I've heard the 8.8 cm Pak 43 and 8.8 cm KwK were excellent guns for the time and could deal with nearly any allied armour in it's day, even giving the Soviet IS-1s and IS-2s trouble. The Pak 44 seems a bit overkill IMO. I'd just like some info from reliable sources and qualified people.
Edit: I'd also like to know why the Germans went with the odd calibre sizes, i.e. 37mm instead of 40mm, 47mm insead of 50mm,88mm instead of 90mm, 105mm instead of 100mm,and 128mm instead of 130mm
When introduced, the Pak 43 was definitely capable of knocking out any Soviet tank at long range. The introduction of the gun in 1943 prompted the development of much more heavily armoured vehicles, such as the IS-3 and IS-4. The IS-2 was not impervious to the gun, but the thickest part of the armour, the upper front plate, could be penetrated from a range of 250-600 meters (depending on the source). Other parts of the tank were more vulnerable.
The IS-3 and IS-4 were much more resistant: the only part in the front of the IS-3 hull that could be penetrated was the lower front plate, and only at 300 meters. The turret was vulnerable at 350 meters. The side was also very resilient, with penetration possible at 150 meters due to spaced armour.
The IS-4 was even more protected: trials of the Object 701 #1 showed that the tank was protected from 88 mm shells from that gun from the front +/- 60 degree arc for the hull and +/- 30 degrees for the turret. The side was more vulnerable than on the IS-3.
The T-54, a prototype of which entered trials in 1945, was immune to the Pak 43 from the front. The protection standards would only be increasing, with the goal for NII-46 to develop armour layouts to protect medium tanks from the Pak 43 completely.
However, none of these tanks made it in time for the war. The IS-3 entered production in May of 1945, but didn't see service until much later. Against designs that were actually fielded, the KwK 43 (and even the 7.5 cm KwK 42, really) was sufficient, especially considering the additional costs of fielding these guns. The Pak 43 was incredibly heavy compared to other countries' anti-tank guns. Considering that it's very important to constantly move AT guns on the battlefield so that they can't be detected and destroyed by enemy tanks or artillery, the Pak 43 was already scraping the ceiling of what a feasible AT gun looked like. The even heavier Pak 44 would not have been useful unless placed in some kind of bunker or offered other protection (like on a self propelled mount).
Sources:
M. Kolomiets Tyazheliy Tank IS-4
P. Samsonov IS-3 Prototype Penetration Testing
P. Samsonov IS Penetration Tests
P. Samsonov NII-48 Experiements, 1946
Y. Pasholok A Story of Failed Unification