I've rode a lot of clumsy horses in all kinds of games, but good god did I almost lose it a minute ago trying to do those damned horse races. I'd have better luck taking a werewolf potion and running it on foot. Is it just me or does Roach constantly go off road and/or come to a grinding halt for every bloody thing? Seriously..

You know nothing about clumsy horses until you've played Two Worlds. Now THAT one felt like piggybacking a friend back home after you both nearly drank yourselves to coma in your least favourite pub.

Yeah Roach does go offroad sometimes. But I've never had any difficulty winning races. The opponents don't even catch up as long as you start sprinting.

Love Roach's autopilot, but l prefer to travel on foot, use horse only as fast travel when needed,

fun fact : Roach is supposed to carry bags, but overweight slows Geralt - so it is obvious who really carries all this swords and decaing moster heads =)

i thought she could be controlled rather easily, especially considering how extremly fast she can run.

Maybe its just me then.. but it really does seem like every damn bump, branch, bush and bird causes my horse to freak right out and hit the brakes like we've hit a cliff or something. Even rough surfaces like old bridges cause this, ack..

Anything good and exclusive from the horse races or may I just skip it?

Yes, you get much better gear for your horse (carrying capacity for Geralt, horse stamina, etc.) by winning various races.

You could skip them (with the exception of one that's compulsory as part of a main quest), but then you'll be missing out on useful equipment and either have to find it by random chance, or purchase it for the same advantage.

***SPOILER***

You can skip them for the most part, as the best saddlebags can simply be bought from a merchant in Novigrad right next to the King of Beggars.

You're fine, horse is fucked.
Roach's behaviour looks like either improperly built or improperly tested. For example, as you sad, she is great bump collector, going from full speed to all "neigh" on plain surface with no obstacles around. In same time, it can rush towards obvious obstacle (a wall, high enough, so she wouldn't be able to jump over) and run into wall, not stopping. given control input lag, she can keep running even after you release "shift" key. Sometimes she can run into people without slowing down, in another time she will stop when there is anyone ahead.
In same time, she can jump over certain obstacles automatically (I haven't pressed spacebar), like small creeks or slopes. I don't really know what is bug and what is feature here, but according to my family, during playing Witcher 3 I already cursed more, than during previous months this year, even while playing GTA V, which was far from puhfect, but at least in most cases was predictable.
Besides, when Roach is galloping on the road and you don't touch directional controls (forward, left, right), her stamina is not decreasing. But as soon you touch these controls, she starts losing stamina. WTF? You can skip it, as I got best equipment as loot or bought from traders. IIRC Zerrikanian saddle (+80 stamina, best so far) was found as loot while rugged saddlebags (+70 carrying capacity) and zerrikanian blinds were bought. So racing is kinda pointless.
Oh, some minor spoilers - if you still want to participate races, exercise manual steering as Roach tends to collect everything on the way, and in Novigrad races beware of third run (by the way, unlike Velen's, Novigrad tracks are different) - track does 8 figure there, and I got my waypoints wrong (one of glitches that started flooding game after entering Novigrad), game sent me back to the part I already visited. Thankfully Roach decided to stop and my opponent got forward, showing me way. :)

Thanks for all the input, and I'm glad to see I'm not alone on this. I think I'll skip what I can from the races and just buy what I need (currently sitting on 14k at lv 13 and no need to spend it anyway). I really like the game and don't want to start hating it over some screwed up horse mechanics.

As I completed all races, I think worst are around Novigrad, mostly because of goofy waypoints, tracks, and Roach's behaviour on them. Velen has fixed track, and for most part are Ok, but random pedestrians can come on track, and there is one "hairpin" turn that causes problem for Roach. Skellige races are simplest.
but outside of some monetary gain, there is really no point participating, save you some nerves you'll spend a lot anyway, while riding your faithful steed. ;)

I had all sorts of problems in the races when trying to use a 360 controller. After switching to the keyboard for the races I didn't have a single issue. I still us a controller for most of the game and switched to keyboard if controls become an issue.