My favorite two bikes of all time are my late model YZ125 and YZ250. Both of which I bought used with low hours, for a small fraction of the price of a new bike. I've ridden a few 144s (none were Yamahas, though), and they are great fun. It all depends on what type of power you ask for. I have a few friends that have had them done by Eric Gorr, and every one of them has been more than satisfied. Ardfarkl on here had a YZ144... maybe he will chime in.

I have given a lot of thought to turning mine into a 144, but I just really enjoy my 125 the way it is. Hands down, its the most fun bike on the market. I probably will send it off to Eric Gorr eventually, though.

The Athena 144 kits aren't very impressive, due to the porting specs (which are almost identical to the stock 125 cylinder, from what I've read). Adds some low end, but is kind of flat on top. With some port work, they rip and are a viable option since you'd then have an extra cylinder lying around.