I am currently trying out Newer Technology's MAXpowr 300/300 G3 upgrade in my Power Mac 7500. I won't bother you with all the benchmark results (because such details are readily available invarious Mac magazines and web sites), other than to say that the processor score was almost three times faster than the already fast 200MHz 604 processor that I had been using.
However, you will not need to run benchmark tests to confirm that the G3 card really speeds things up. You'll notice it from the instant you turn on your Mac. Everything you do takes less time, sometimes dramatically less. Even disk intensive activities, such as launching applications, go faster. From format changes in Word to applying filters in Photoshop, the key word is "speed."
So is the upgrade worth the cost? The 300/300 is the top of the line. Saving time would really have to save you a lot of dollars for the approximately $2000 300/300 to be worth its cost. But you'll see almost as much improvement in its less speedy (and much less expensive) G3 cousins. For most users, a less expensive G3 upgrade makes more sense.
I admit that my 200 MHz 604 is still more than adequate for most of my daily tasks. Your Mac may seem just fine as well. Still, any user that is not yet using a G3 processor (either via an upgrade card or a G3 Mac) should strongly consider getting an upgrade. If you can afford it, you won't regret it.
Note: Last September, I reported on using a MAXpowr Pro 266/167 G3 upgrade card. In looking over the item, I found that my reaction to that card (even the words I used to describe it) was nearly identical to today's description of the 300. I guess there is something to be said for consistency.