If your timing belt goes... you will gain a new car :)
I waited until 145,000 before I changed mine but I would not do it again- it is worth the peace of mind knowing it won't destroy your engine
just did mine today (106K), but on the 98 es300 the timing belt can break and it won't do any damage to your engine. if you search these forums there are threads about how this generation of toyotas/lexus engines don't really get hurt if the belt breaks. you'd be stranded, but it shouldn't damage your engine.
I called Lexus in Pittsburgh yesterday & they assured me that my ES-300 3.0 WAS NOT a INTERFERENCE ENGINE and that I could go to 90,0000 miles. (Thanks for help.)