Best bang for the buck by far is the XT Racing lap timer. It keeps track of session information, has an "auto-on" feature (I use that a lot), you can pull down the info onto your computer and export it to Excel down the road, etc. I had a MyChron Lite and sold it in favor of the XT Racing timer. Another plus of the XT is the fact that it will work off of the MyChron beacon.
If you are going after advanced functionality then MyChron makes some better timers out there (multiple temp sensors, RPM, etc). It can completely replace your stock display. However, they are pricey ($400 and up) and if you crash the front end then they typically get destroyed. I have a friend who picked up a MyChron 3 (I think that was the model) and got to use it for 1 weekend before he lowsided, took someone out, and destroyed the timer. $400 down the tubes...
BTW, my XT timer was in a 100MPH lowside tumble and a 60MPH high side and came out without a scratch. The MyChron has a plastic case where the XT is housed in aluminum. The XT is virtually bulletproof.
It also runs off standard AA batteries where the MyChron Lite runs off watch batteries.
