I disagree with the misleading answer of Gordon Allen which does not even answer the question.
The German flak crews were extremely effective and by the end of the war, 80 percent of all Luftwaffe resources were being dedicated to AA defense.
The there were only 29 women who received the Iron Cross in WW2 and some of those are anti-aircraft crews while most of the others were nurses. As the war went on, the proportion of women AA crews increased dramatically as the need for men in combat rose. At first, women were assigned primarily to searchlight teams as searchlights were critical to shooting down night bombers. A bomber caught in the beam of a searchlight was usually done for.
By the end of the war, the Germans were employing 13, 748 searchlight systems that were attached to listening devices and radar and automatically tracked aircraft.
The searchlight was a critical part of AA defense, A plane caught in the light was almost always a dead duck.
There were all kinds of anti-aircraft guns made during the war, the most commonly recognized being the 88mm flak. As bombers went higher, the Germans also developed the 128mm flak gun. All of these models were extremely effective and almost 30,000 of all high caliber types were produced and stationed in mobile batteries, as well as fixed positions, around high value targets. While the Allies pioneered the proximity fuze, the Germans had a similar mechanism but for most of the war, the crew had to adjust the fuze based on the assumed or determined range. Although the 88 and 128mm were like “rifles” and accurate on ground targets, the goal of the flak gun was to fill the sky with hurtling pieces of shrapnel that would tear into planes and damage them til they crashed. An 88mm gun required a crew of 8 people to operate the gun, though of course it could do with less but the rate of fire was impacted. Note that each 88mm cartridge was 32 pounds, so operation of the gun was well within the ability of women to manage and operate the weapon.
While the British actually had a similar gun to the 88, the iconic German model has been memorialized as one of the most effective weapons of WW2. It was capable of being effective as an AA gun, as well as attack ground targets, and in desperate situations, b used as artillery, though this would ruin the gun.
The 88mm gun was also excellent in the tank killing round and was feared by enemies because it could destroy targets at great distances but this capability is beyond the scope of this question. Near the end of the war, Heinrici was put in charge of the German Armies on the Oder in a desperate attempt to stop the Russian advance and he was ordered by Hitler to use immobile, concrete emplaced 88mm guns as long range artillery but Heinrici hotly contested this because the artillery rounds damaged the barrel lining and made them useless for anti-aircraft or tank defense, but this was another battle he lost and the guns were ruined firing ineffectual artillery rounds at the Russians, but I apologize that this is off topic again.
The Germans had a wide range of AA guns. The Quad Flakvierling 38 was the apotheosis of this development. This gun was focused on shooting down ground attack aircraft as opposed to bombers and was very effective in this role. Before rapid fire gatling guns were invented in the 1950s, this weapon had an incredible rate of fire that would easily destroy any fighter. The gun could be trained manually or electrically.
Towards the end of the war the Germans put flakvierling on the bodies of damaged Panzer 4 tanks. These weapons were widely sought by infantry regiments and were very effective but they were made in too few numbers to make a difference.
The 88mm gun was considered so effective in its role that 11 nations adopted captured units for their own use after the war, In addition to the high-calliber flak guns, the Germanss developed almost 200,000 lower caliber 20mm and 37mm anti-aircraft guns during the war, which like the 88 could also be used against ground targets. Over 19 different models were made during the war, the quad barrel units could theoretically fire 1200 rounds a minute (but since it used a 20 round box magazine, the practical rate of fire was about 1/4th of the theoretical rate. Range was up to 2200 yards but these types of weapons became less effective at distance. To make up for the capabilities of the 88. there were various kinds of ammunition used to destroy planes and locate them, such as phosphorous and illumination rounds.
The Flakvierling used a standard 20 round magazine. As materials got scarce as the war progressed, these magazines became increasingly flimsy.
The Soviets lost 110,000 planes during the war,, 60,000 of which were lost in training accidents, ground attacks, maintenance failures and the like, meaning 50,000 were lost in combat. The Americans and English lost about 88,000 aircraft of which about half were lost due to training, maintenance, ground attack and other non-combat related losses. Of the actual combat plane lost in the war, After the war, the Americans determined that half of all combat air losses were due to flak.
The 88mm gun used a stereoscopic range finder for ground targets. In fixed positions, the guns could be attached to radar or listening devices and traversed and aimed electrically.
Note that as the war progressed and the Allies bombed more German facilities and cities, 50 percent of all German fighters and air defense assets were moved back to Germany. Flak protection was an artifact o the Luftwaffe, as opposed to the Wermacht and as a result there was often poor co-ordination between the army and air force of the German military. Col.. Hans von Luck tells of being at D-Day in his tanks and comes across a battery of 88mm guns that are inactive, even as English tanks are moving off the beach. The commander tells Luck that his only job is to shoot down planes, so Luck threatens him with a pistol until the crew starts to destroy the tanks, which blunts the British attack.
Engaging ground targets was where the 88 really shined and developed its fearsome reputation but the gun had a very high profile and was hard to evacuate to crews were often killed in action while utilizing the gun in this configuration.
During Operation Bagration, the Luftwaffe was caught with their pants down and unable to get more than a handful of planes into the air to blunt the Russian advance – but the German ground flak shot down over 800 Russian aircraft. (Of course, in the end, the Russians wiped out an entire German army group and the Germans SHOULD have surrendered in that moment).
As the fortunes for Germany turned against them, they moved more and more assets to Germany to protect factories. Batteries of guns would be set up to allow saturation air fire. A “kill” would be recognized by a “kill ring” painted on the barrel.
The Germans built massive flak towers that were armed with both 128mm and 88mm flak guns that were deemed extremely effective in every role and were virtually indestructible. SS Sgt Willi Rogmann tells of visiting a flak tower in the last days of the war and seeing a squadron of Russian fighters attack the tower, only to be wiped out by the quad flakvierlings on the platform. The 128mm guns on the tower were able to vaporize Russian tanks from great distances. Some of the flak towers are still there.
The Germans built massive, invincible flak towers around major cities in 1940. These towers provided awesome defensive firepower and held out till the bitter end. Some of them are sstill in existence because they are really difficult to demolish.
While it was rarely used on vehicles, the 128mm flak gun was easily the most powerful and advanced gun of its type in the war on any side. It could propel a 50 pound projectile to 45,000 feet. The Germans were unable to mass produce this gun but it was emplaced on flak towers with great effectiveness. It could reach any bomber because of its range – and high altitude to escape flak means the bombers are unlikely to hit their targets – even with saturation bombing.
In conclusion, I posit that flak guns used by the Germans were extremely effective in WW2.