you can read alot of user experinces with vapochills in the forums @
www.vapochill.com
they basicly work the same way your refrigerator/home or window airconditioner would with the main diffrence being that rather than having the evaporator cool the chest as in a fridge, or the air that would pass over it in an airconditioner it cools the cpu.
the vapochills and prometias are a little more solid than a traditional tec setup imho. they come packaged with the proper gaskets and dielectric grease as well as a heater for the clamshell (the piece that holds the evap head to the cpu) and the rear of the motherboard directly behind the cpu socket.
the drawback to these are the prices that come with them. a brand new typical vapochill ls will run you around 820$ usd. if you search around you can find used prometias and older model/style vapochill units that are on ebay and various oc orientated forums like the classifieds here for quite a bit less.
the temps you see from a vapochill to a mach I or II can vary dependant on what cpu you are cooling with it, what type of refrigerant is being used in the system, the airflow crossing the condensor coil and compressor and ambient temperatures in your room. typically people will see apprx -40c to -50c on the evap head and anywhere from -15c to -25c on the cpu (this varies with load as well and is just an approximation)
all and all with a powerfull enough tec solution you can get to alot of these same temperatures the biggest factors there would be the proper sizing and power of the tec (an example would be a 80w tec on a highly oc prescott with crappy psu wouldnt help much at all) and the ability to remove the heat generated by the hot side of the tec effectively enough.
the thing you may need to think about though is as gl4ss mentioned the tecs arent known for efficency and can run your electric bill up at a decent pace.
since ive installed my vapochill ive run it for the most part 24/7 over the past few months even with winters onset my electric side of my power bill has only gone up like 10-15$ which isnt bad at all for my neck of the woods maybe someone who runs a powerful tec (like 226w ish) or runs multiple tecs could share thier power usage costs with us.