I have a Opteka grip on my D3000 and a Zeikos grip on my D90. Both are made out of plastic. I have never had any trouble with either grip for the past 2 years or so.
How ever I have had troubles with the non OEM batteries that came with the Zeikos grip on my D90. They are cheap and don't last and don't hold much of a charge.
If your thinking about a grip spend the money and get a set of the Nikon batteries.
As for the battery door, I felt the same way the first time I did it, now it's second nature to take it them on and off if needed.
Open you battery door about half way and give it a little pull at the angle it is at, it will pop out just like a tooth, to put it back on position it at the same angle and slide it back in.
One nice thing about the grips, you can store your removed battery door cover inside the grip so you never lose it.
That my experience with third party grips so far.