1. How is the network support?
Sockets are not better than the networking in B3D/+. It's the same as comparing raw OpenGL to B3D's gfx engine. They are raw socket commands, as opposed to B3D's nice wrapper functions. That said, they are usable, they work etc. So there's no reason you can't write similar wrapper functions, but, you know, there's more coding involved.
2. Opengl, how is it?
What Noel said.
3. GUI, how is it?
It is more or less the same as B+, with a couple of small things gone and a couple of new small things. Oh and the mammoth task of running the same across three native platforms, natch. To stress the B+'iness of the module : the win32 module is still using the C code from B+. So, if you know what B+ is like, you can decide for yourself. =]
How about the drawbacks of going from Blitz3d -> Blitzmax?
Pretty much already discussed, but B3D is a stable, finished product - you know in advance what you're getting into. Max has had semi major improvements made with the last two releases - requiring mostly simple changes, unless you wrote messy code. Also there's always room for the introduction of bugs with such huge changes. Also in some ways Max is less developed than BlitzBasic (socket support vs networking support, freesound vs cut down FMOD etc) so some features, if you require them, will take extra coding/research. Of course Max has a tonne of benefits as well, but you didn't ask about them ;]
I have no experience though. I don't have time to code. I spend too much of it on these forums. =]