I am a veteran with X-Com (I bought it for the Amiga version, the terror missions took ages to load...), through Apocalypse and the latter Aftermath and so on. I find UFO AI a great game, really devoted to the spirit of the original game and the most amazing thing is that it is developed by volunteers and with a freeware license - I'd like to help in translating it in italian once I'll have completed my first campaign.
So far, the only thing I really don't like is the way reaction fire is implemented. I've read and understood how it works in 2.5, but to me it is a non-sense. Let me explain.
Theoretically, when saving TUs for RF, it means that I'm using only part of the actions I could take in my turn for using them later. So my soldier is basically on guard, scanning for movements in front of him; this means that he should be ready to fire.
In the way it is implemented now, instead, my enemies can perform many actions, and I could not be able to fire at them: for example coming out from a door (no TU as they appear), take another step in my direction (2TU), then back (2TU)and then again in the room(2TU). Even with a simple snap shot I couldn't have the chance to fire at them; so, in the end, I'm penalized for not having used all my TUs in the previous turn (I could have thrown a grenade instead, to maintain the same example).
Having a sniper in reaction fire mode is useless: it is almost impossible to see an alien fired with an aimed shot, and the snap shot is unaccurate even at close range.
In X-Com (I know, it is a different game) the soldiers shooted as soon as an alien walked in front of their LOS; as far as they reserved TUs in the previous turn for the selected shot, they were able to fire immediately, and I feel it is the right way to use the RF factor. The only exception I can understand is if the target is "quicker" and has an extra movement allowed (I see you while turning to your direction, I jump in cover). This could be handled by the soldiers/aliens stats.
Another thing I'm not able to understand: how is it possible that the aliens can RF even if I'm not in their LOS? I could understand they shoot me after I've fired and they survive... but before? When shooting at their backs? Is it a matter of telepathy?
Ok, this is my two cents. I hope this could give ideas and a different look at the aspect!