YOu don't get an upgrade like this for average utilization. You get it so when you're processing something in Photoshop and running a VM the machine is still usable, if marginally so. My CPU cores hover around 25%, but every once in a while when doing something intense they get pegged at 100%. That's when you want an upgraded CPU.
While there are exceptions, it is generally the case that new software has new features that need more cycles. Snow Leopard is one of those exceptions. Nonetheless, it's essentially a 10% increase in performance for a 12.5% increase in price.
It's worth it to some, not to others. for me @ $100 it's a no brainer. Especially in light of all the stupid crap I spend well over $100 a month on...
Well, I'm obviously not going to argue that a slower CPU is better than a faster CPU, but it's misguided to think that 10% is going to be the difference between "sluggish" and "responsive" or "obsolete" and "futureproof."
If your Mini is under enough load to the point where it's sluggish and it takes 1 second to, say, switch from app A to app B, I frankly don't think it will make a lot of difference in your life if it took 1.1 seconds instead. You might not even notice the difference. (And again, just because you are doing some "processing" in Photoshop and running a VM doesn't mean your Mini will be under a lot of load... I'm running a VM right now and it's taking less than 5% of the CPU power of a single core.)
I will freely admit that in some situations it makes a lot of sense to get the upgrade. For example, my friend works at a software company that uses Minis as build machines and it can take a couple hours to finish a build. If the build finishes 10% faster, that means that several developers each potentially gain an extra 12 minutes of productivity. Definitely worth the upgrade at twice the price. But if you just have a hazy notion that you might do some "processing" in the future, then for god's sake, save your money.
Of course, this is the MacRumors message board, where almost everybody here thinks that if somebody has any money left over in his bank account, it's because he didn't buy enough upgrades. Sure, the upgrade is "only" $100, but surely you can think of something better to do with that $100 than spend it on an almost trivial CPU upgrade that you won't take advantage of 99% of the time and might not even notice or appreciate for the other 1%. I can think of a dozen other things I'd rather spend $100 on right off the top of my head, and even if I couldn't, I'd be better off with that $100 in my retirement account.