On it's own it may not be pretty, but it certainly works!
If you're a power user like me you never seem to stray too far from your ThinkPad. I use mine for more hours a day than I honestly care to admit. Battery life is of great importance to me. I've never been a big fan of the extended batteries we offer, not because of the function they provide, but because they make my ThinkPad look less elegant. They certainly do deliver on better battery life, and some people may not mind the bump. Being a design centric kind of person, I prefer my ThinkPad to be bumpless. Enter the bay battery. It neatly replaces the optical drive for my T400s and gives me a huge bump in battery life without the physical bump. I just recently made the transition from onboard optical to the bay battery and have not regretted the move. I rarely used the optical drive other than to show someone it had one. I am probably not the typical user. I really like it. Yes it makes my system a bit heavier, but that's a tradeoff I'm willing to make. I haven't measured the exact battery life I am experiencing, but our marketing claim states that adding the bay battery may take total battery life up to a impressive 8.5 hours for a T400s. Your actual battery life depends highly on actual CPU use, screen brightness, fan speed, etc. etc. etc. Suddenly I feel like a lawyer. If you are hungry for battery life, but don't like extended battery bumps, I suggest you strongly consider this option. It works for me.