I worked with a personal trainer during my last months in London, between October 2010 and January 2011. When I started, I had been cycling to work most days (a bit over 3 miles / 5 km one way), and was eating what I thought was not badly (well... it wasn't, quality-wise, but the portions were too large and there were occasional excesses on sweets beyond what I could afford), but hadn't seen any weight loss from a start at nearly 230 lbs. I hadn't done any organised sport in maybe 20 years, had never been to a gym, wouldn't know where to start using the machines, thought fitness wasn't for me. I'm also not naturally coordinated.
So when I signed up for a gym I also took a special offer of a few sessions with a trainer. The gym (a large Virgin Active in London) hooked me up with a young man called Jason, and it was a great match. (First important point: you need to be comfortable with your trainer's style.) He was immensely helpful: he pushed me more than I would have, set me a reasonable but doable strength training program and *simple* effective cardio. During the time with him I went down nearly 30 pounds, and he was very proud of me.
What I didn't work on with Jason but have seen others benefit was nutrition. My frank opinion -- and the following is full of assertions I'm not backing up by much -- is that it is really quite hard to eat the right amount of calories and the right stuff. Most overweight people don't seem to overeat by very much, so like myself it may be hard to see what's wrong with the eating. And almost worse, once people decide to lose weight, I see a lot go for an excessively low 1000-1200 calories and strenuous exercise. I also think it's harder for a woman as our margin of error tends to be smaller. I used (still use) Myfitnespal for calorie counting, read about BMR, listened to the (slightly dogmatic but useful) Fat2Fit Radio podcast, and ate 1400 calories or thereabouts, plus a little extra on high exercise days and once-a-month occasions for about 8 months before upping the calories a little. I've seen others do similar programs supervised by a trainer with great success.

