It doesn't work at all well for me and the feedback in the official forums confirms I'm far from alone in this. Perhaps there's some hardware factor, such a duel core processing, which effects horse control for some and not for others (there is/was some issue with day/night cycles and duel core).
I think you may be right... it may be a problem based on hardware. After reading this thread I tried horse riding (I'm not playing the game yet, but my son is) and it works great for me... as long as I follow the steps that have been outlined here and in the manual.
By tapping the 'forward' button once my horse starts a slow walk... and it is very easy to control him. Tap the 'forward' button again and he speeds up to a slow trot, and is still very easy to control. Continue in this way and he will eventually speed up to a gallop at which point he becomes more difficult to turn... and like stated elsewhere in this thread, that is realistic (which I personally like).
Tapping the 'back' button then slows the horse incrementally... or you can push the 'stop horse' button for a hard, complete stop.
For me horse riding is implemented better than I have seen it in any game. All other games treat it basically as a faster form of the character running... which is nice if that's all you want, but it's nothing like actually riding a horse.
So I can see why the general populace doesn't like horse riding... it simply doesn't respond the way they want... which is "even though my horse is galloping I want him to turn on a dime".
But for those who understand the mechanics of horse riding who are still having troubles, I believe you could be right that it may be due to different types of hardware that the game is being played on... for reference here are my system specs:
Intel 3.73Ghz Extreme Edition processor (single core, multithread)
3Gig Memory
GeForce 8800GTX
2 x 180Gig RAID1 hd where my pagefile exists
200Gig external USB hd where the game is installed
Sound Blaster Audigy 2ZS
Sony DVD drive
22inch Samsung widescreen monitor
Game runs like a dream in 1600x1050 resolution with all details at max