I hate enemy respawns like COD. It annoys me and ruins all kinds of tactics. I played the original and there was a set number of enemies which I appreciated. I think the aliens respawned a little though.

On a user review someone posted that there is a 'slight' respawn of enemies. Any truth? Is it annoying?

Are you talking about the campaign? I am about half-way through, and I have only noticed something like this once. There is a level where you need to make a distraction so a civilian can get by a CELL checkpoint. You have to go to this church and set off an explosion in these two explosive storage areas. I set the first explosive, got out and hid behind a shipping container. After the explosion went off a CELL guy spawned right next to me as part of the reinforcements that were showing up to investigate. But once I killed all the new guys they don't respawn.

The only annoying thing about that was that I spotted it. It is not a perpetual respawn like in COD games. It is event-specific and even then it only spawns in a few guys ONCE. So, I would say if you are worried that it's gonna be like COD respawn, don't be. It is not going to be annoying.