I waited a few years to get one. If I had to go back and do it all again, I would have ponied up the cash from the get-go. It's not a huge thing, but sometimes tweaking the little things makes life easier.
I can see from far away how my boil-off is progressing. This changes all the time due to wind, propane tank fill level, outside temperature, barometric pressure, etc. There's no way to know for sure exactly what rate things are going to boil off, and this way I don't need to check with a dipstick every 10 minutes or guesstimate--I can see from 10 feet away if I'm closing in on my final volume too quickly, and reduce the heat, add water, etc. before it's too late.
Usually being a quart too high or too low at the end isn't a deal-breaker, but you have to also consider that the final volume affects the gravity as well. Some recipes are on the edge for style guidelines and every variable you can pin down helps.
Plus, the sight glass makes filling the kettle with a hose a breeze, and you can add o-rings that slide up and down the glass and serve as indicators for where you want to be at different stages of the process (strike water, sparge water, pre-boil volume, etc.)