I have a Bi-Orb, and they're pretty good. I would only recommend keeping 2 goldfish or 5-6 small tropical fish if you get a heater (something like Neon Tetras or something of a similar size which don't grow above an inch or so) - because the surface area at the top of the water isn't huge and that means the water does not get oxygenated as much as a normal tank. I also considered getting a pair of
dwarf puffers as they have so much character, but they're harder to look after and can be difficult to find!
After 6 months of so of having the Bi-Orb, I bought a slightly larger standard shape aquarium for tropicals and put a few goldfish in the orb. I'm now planning on getting a much bigger tank for Cichlids or Marine fish - I'm addicted!