I have an '04 and '05, neither with the oil mod at the moment. The reality is, you may get 100 hours before it rears its head, or 10. No one knows. If you don't ride the rev limiter constantly and keep fresh oil/filter in it I would still get it done but it wouldn't keep me from riding in the meantime.
Otherwise maintenance is key. You NEED to keep the air filter clean/oiled, fresh oil, and stay on top of valve adjustments(not to hard, maybe a 15 minute job once you have doen it a few times.) The motors really are pretty durable.
A few known issues: There is a plug in the top of the motor on the '04 that is rubber and can blow out if you overfill the oil. Replace it with a metal one if it hasn't been done already.
'05+ already have a metal one in place.
The charging system doesn't run full time on that model year as well. Running with the lights on makes the machine constantly charge the batt and solves the issue. If you don't have lights, Yamaha has a resistor kit that likewise solves the issue.
I hate the chain adjuster on the '04/5 as well. It is functional, but the '06+ style is WAY better. I switched mine out, but it can be spendy. You basically need the swingarm, brake, brake plate, shock and linkage to make it work. I bought everything off a guy doing
a part out.