The UI isn't that bad. Anyone who says they quit playing the game because of it is missing out. It could certainly use some improvement though. Making the fonts smaller so that you don't have to scroll so much would be a start. I don't think we're going to see as many UI mods as Oblivion and Fallout 3/NV. I extracted the files and they're in swf format now instead of xml. As far as stability, the game has been very stable for me. The only crashes I've had are from alt-tabbing out of the game since the overlay isn't working.
The game auto detects my system and set the graphics to high. I bumped them up to ultra and it runs fine with no tweaking. There have been very few areas that I noticed a slowdown, usually in a town where I'm in a place overlooking a lot of the city. My system will be 2 years old next week. I've only upgraded the video card, HD and RAM since building it. I'm not overclocking any of it either since I haven't found a game that it can't run maxed out. I don't see why people are overclocking their systems and running overclocked multi-GPU setups when my 2-year old, stock 2.66GHz Core i5 750 with a GTX 470 runs every game that's come out in the past two years on the highest settings.