It really does depend on what you are using it for to be honest...
Some pointers though...
Noise- go for a fluid one rather than fan and a slick (not necessarily turbo specific) tyre
Heat- you'll get warm v-quickly so have a fan to hand if you can, and a towel for that matter...sweat-tastic
Boredom- you'll get bored quickly (if like me) so dig out a video to watch, maybe something from the 'sufferfest' range as they kind of give you something to push for when on it
They can be good for those crappy days when going out is not an option but you will need will power to go out on it as its not as exciting as the real thing.
Worth trying though and loads out there either new or second hand...even ones where you can plug into PC and do real XC or road stages but they are top dollar.
Cheers