Odyssey has cool stuff, but the customer service is horrible. Heard too many complaints about the Halo's breaking and no replacements available, from too many shops, not just owners. ICD has GREAT customer service, they will even fix guns they no longer make (like the desert fox).
A good anti-siphon setup and good reg will do you right in most guns. PPS, AKA, and Air America make some of the best, and most reliable plus service is great. If you go with an AKA or AA reg, let them know you are using CO2, they will give you harder reg seats that are friendlier to liquid CO2.
Now here is the important part, that most people wont tell you. You may not be able to switch gasses. Let me explain.
I have my main auto-cocker set to run 350-400 psi from an anti-siphon CO2 tank. it shoots within +/- 3 fps with decent paint, better with great paint. Attach my HPA tank, and it leaks like a Oldsmobile driven off a bridge. PPS tells me that it is because of CO2 being a more dense gas.
The only time I have trouble with that setup is in extreme cold, like when it is in the single digits. I get frost to hell and back on the stabilizer, and a little shoot-down. in good weather, however, I can get 3/4 a case off of a 14oz tank. Heck of a lot lighter than my fiber wrap crossfire 68/3000, and cheaper too.
Now I wont count out using a Smart Parts max-flow on CO2, they were originally designed to run that way. While great regs, they do require a little more maintenance, so buy a nice big tube of "shocker lube" or what the rest of the world calls it, Dow33 multi purpose lube (available for 1/2 the price through grainger and I think mcmaster.com)
OK, enough of my rambling....
BKO - Good guns, efficient, run great. No bad price either.
Dragun the one- good idea, customer support very lacking (or so I have been told)
O3 - Supposed to be good, questionable service.
One of the fines electro's you will ever find is an AKA viking. Top notch machining, solid feeling, great service, great guys over there to deal with. No longer made because of certain other companies pursuing lawsuits, but parts still being made and one of the fastest guns out there. You can find them for decent prices used, but will still be in the 400+ range.