Riding a fixie doesn't mean you'll lose weight faster. I haven't lost a lb. riding a fixie and I could easily stand to lose 60. The reason I haven't is I don't ride enough. I should be riding over 100 miles a week w/ my HR in a target range for at least 40 mins to an hour, and watching what I eat. Instead I'm in the 50-70 miles per range usually just cruising along and while I'm eating sensibly, I'm not eating sensibly enough.
The benefits I've seen so far are my spin is better, climbing is better and my legs have more muscle. So when I jump on the mtb or roadie, I'm an exta mile or two faster than I used to be on average. Plus it's fun!