No experience with 3rd party grips. In fact I've never sprung for one at all before. But I'm committing myself to the D600 and bought a MB-D14. It's great. In fact it feels better built than the actual camera!
I notice the controls are a bit sticky/stiff and the AE-L/AF button is recessed a bit making it tricky to locate but I'm sure with practice it'll become more intuitive. Overall I like it a lot. It's grippy and fits the base of the body snugly.
After the sticker shock passed I'm glad I went for the Nikon grip.