The craftsmanship of the bike is amazing! But it's one of those things that you have to see for yourself and decide.
The MSRP of a TM 125MX is $7,499.00 which is high compared to the others, but you need to look at the whole picture. These machines are build like works bikes and have the best components as standard equipment, then it doesn't seem so bad.
Where are you located? You are welcome to take a spin on my bike if you'd like.
All that said, they are not for everyone. They're really built for serious racers.... if your looking for a trail bike, you probably wouldn't be happy.
As for the engine trouble that you speak about, where could I check up and read the issues these kart racers have had?