I voted that I sometimes use 3.0 rules after converting them, but that needs qualification.
I'll admit that I took my 3.0 DMG, pulled out the NPC builds and apprentice-level rules, and binned the remains. My 3.0 PHB and MM are in a pile to sell. I haven't looked at, and don't intend to revisit the core rules.
For a writing project, I've used the 3.0 SRD for one specific piece of information that wasn't brought over.
I occasionally use material from the class books.
When I have time to look seriously at psionics, I'll be rereading the 3.0 Psionics Handbook.