I had heard that the final 20 minutes were just destruction, before I saw MOS.
But when you watch the Supes vs Zod fight, it's barely more than 5 minutes.
I know that some people complained that the last half hour had so much destruction and noise that it wasn't enjoyable.
For myself, I thought it was great, Nolan and Snyder set out to show us what it would look like if Superman got into a scrap in downtown, and that's exactly what they did.
I have no idea why people complained.
Personally, I thought the Battle of Smallville set a new high in terms of Superhero vs Supervillain fights, and was the best action piece of the film. Now that was just magic, in terms of its brutality and pace.
Supes vs Zod was good, and I liked the speed at which it took place, in fact it goes so fast that there are lots of little things I missed on the first viewing. As far as fights go it feels a bit shapeless, well compared to Smallville anyway (maybe cut it by a minute and have a bit more choreography of them going toe to toe, although I loved the exchange of blows in orbit, nice, very nice - and I guess flight was an important part of that fight, as they wanted to show Supes fighting someone on even terms.
So, all in all, I had no problems with the action.
BTW I liked that Krypton didn't just blow up, the Kryptonians were responsible for their own demise, and I really liked the brief Kryptonian Civil war - love that shot where Jor El runs out of the council chambers and just stops and looks, his jaw drops as he sees that all out war has erupted. I call that the "**** gets real on Krypton" shot.
As an opening action sequence it set the pace and tone for the film. Nice Snyder, very nice !