Before this year, I took about 5 years off from riding and MX racing. Previously, my favorite bikes were always 125s. I've owned various 125s, 250s, and a CR500, and the 125s were by far the most fun. When I decided to get back into riding, I also debated between a 125 and the new 250f. I decided on the 250f mainly because it sounded so good, and because it's the new kid on the block. It's always fun owning the bike everyone else wants to hear about.
Throughout this year, even though I've been having a great time on my new Yamaha, I was still debating whether my next bike (2003) will be a 125 or 250 4-stroke (maybe a CR125 or CR250F?) This past weekend, after a friend let me take a few laps on his CR125, my eyes were opened! I will never go back to a 125. My 250F is better in every way! Way more power, way better suspension, much more traction. The CR did turn better than my Yamaha, but I plan to work on that. Once up to speed on the track, the CR did not feel much lighter than the 250f, probably because the Yamaha's suspension it so much better.
Maintenance: In the 4 months I've had my 250F now, I've only had to change the oil, filter, and air filter every few rides. No other maintenance issues. No starting problems here.
I found another benefit that nobody seems to mention. The YZ250F only uses about half the fuel that a 125 uses, and it doesn't require expensive two-stroke oil at $5+ per bottle. The net effect is that your fuel costs are 1/4 that of a two-stroke!
Hope this helps.