Well, I use a lot of weapons and accessories from BBI and PTE (as well as the great guys at Marauder Inc). PTE is definitely my favorite thanks to the nicer scale of the weapons (the BBI guns are a bit larger than they should be), as well as the inclusion of gas cans, oil drums, ammo boxes, barbwire posts, tank traps, etc. with all their products. You always get a good bang for your buck with PTE.
I use a LOT of vehicles from Chap Mei, PTE and Corps, mostly to replace Joe vehicles that either don't interest me or that I can't afford:
PTE Little Bird
PTE Buggy (replacing the AWE Striker)
Chap Mei Chinook (replacing the Tomahawk)
Chap Mei large tank (replaces the MOBAT/Mauler and is used as the Joes' main tank)
Corps small one-man gyrocopter (fills out the FANG force)
Corps patrol boat
Corps mini-jet
And, while I don't have it yet, I plan to use a Corps large copter instead of the Dragonfly, mainly because of its smaller size that makes it easier to store, as well as the fact that it's more solid than a Dragonfly.
I've stayed clear of BBI vehicles, because of the insane prices they go for, and I hear they aren't very durable.

That's about it, I think.