I bought a (very lightly used) MBD-10 soon after getting my D700 last year. I hadn't really tested the extra battery capacity but recently invested in 8 black Eneloop rechargeable AA's in readiness for a holiday in Spain.
With an EN-EL3e in the D700 and 8 fully charged black Eneloop X's in the MBD-10 I've just managed a whole week's shooting (500 shots), the camera had a GPS unit attached and switched on all day. With some reviewing, deleting, and full daily back-ups the battery symbol still shows full charge. In comparison my wife got through 3 EN-EL3e's with my D200 (without GPS) for 600 shots (with lots of reviewing of images).
For me the knowledge that I can head out each day without worrying about spare batteries is worth the effort carrying the extra weight around. The only downside is that I feel more vulnerable in unfamiliar locations as the D700+MBD-10 attracts much more attention than the D700 alone.