I'm completely satisfied with it. It's done everything I've needed it to do. I bought it from JDS. I'm sure there are better amps, but they cost a lot more ($1,000+).
I just bought a K340 400 ohms, and I don't know if O2 will drive it well. But I can always buy a more powerful adapter ($10). If you decide to get HD650, you should do that.
Are you thinking of O2 or O2+ODAC?