two of my buddies run them (yz426/dr400) an they they thought it made quite a differance. one of them made his own from a template he got off thumpertalk. if your somewhat handy im sure you could do it.everything else i have read says its worth the money.
On my '02 CR250, it made a big difference low to mid, much more than the pipe/silencer combo. I can't tell for sure if I lost a little off the top or if I am just revving out quicker because I get through the mid quicker. (a couple of the SX privateers here said they lost top, I can't argue) I can say that there is a double out of a turn that I could only hack without it, and with it I downside every lap. Most of my thumper buddies run one and like the difference, only you can decide if it's worth the $ to you.