I have one ST, a .45. It's very powerful, and obviously a well built engine. The downside (as others have commented) is that it requires very precise tweaking to get it just right. On my H9 Twist it's difficult to get the right balance on the idle, to achieve acceptable pick-up without the idle being too fast.

One other thing I don't much like is the very short distance between the prop disk and the needle valve. Others may not care, but personally I don't like adjusting a screw that's an inch or less from a one foot blade turning at 12k rpm.

In summary:

pros - good design, very powerful, good price

cons - not the easiest to adjust, needle valve close to prop, huge ugly muffler (though it sounds pretty good in the air)

It's a good engine but I'm not sure I'd buy another one. If I decided not to pay OS prices, I'd more likely look at an OS clone - probably ASP in Europe or Magnum in the US. I'm always hearing good things about them, and I know that over here I can get good support for them from Just Engines.

Matthew