There is no bike more fun than a 125. More powerfull yes, more fun no. Some guys are 450 all the way man, and refuse to even consider a less manly bike.
Those that are honest about it will ride them at the track and be hooked on the fun. I was and my Honda 450 riding buddy is too. Not instead of his 450 but in addition. He wants a CR125 though. Too bad I can not talk him into an RM or YZ......
I just took my rm125 offroad btw and I can say not once was I wishing for more power even down low. In fact I was disapointed. I was hoping for more clutch work to strengthen the arms for MX. Almost never used the freaki g clutch. In fat I may have use dit more on my heavy handed torquey trail bike.
I Just blasted through the trails in second gear with an occational third gear romp, touched the cluth like 5 times in 5 miles.
More torque would have made it a bit easier to do things like lift the wheel when going slow etc but I could bomb through the single track as fast as I could on my trail bike.
Oh, my 450 buddy rides the 450's and sells them at about 85 hours right before rebuild/grenade time and buys another. I simply do not trust used 4 strokes and every used one was owned by a little old lady from pasedena too.