What are they for?

[Josh220]

D300 grip:
Nikon | MB-D10 Multi-Power Battery Grip | 25359 | B&H Photo Video

D90 grip:
Nikon | MB-D80 Multi-Power Battery Pack | 25345 | B&H Photo Video

It's pretty easy to see why you need to remove the battery door for the D90, and not for the D300.

 

[NateWagner]

As far as brands you should avoid, I'm not really aware of any. Some people complain about the Targus brand. I personally use the Adorama brand which works quite well, and is very solid. I have had no problems with it whatsoever and the extra money in my pocket is definitely nice (compared to if I had purchased the Canon version)