I also own a Vox AC10 with the regular Celestion VX speaker, so I'm comparing this one with the upgraded Celestion Greenback to my regular one. At this point, I've played the new one at 4 gigs and multiple rehearsals; the speaker has about 40 hours of playing time at volume, so it's broken in. The Celestion Greenback is a serious improvement over the VX. Tighter and slightly more prominent lows, warmer mids, and similar but glassier highs. Overall, I'd say the improvement in sound would absolutely be worth it to buy the speaker aftermarket. Fortunately, Sweetwater has taken care of that for us, and at a much discounted rate.
If you haven't heard the AC10 yet, this is a WONDERFUL amp. Much, much better sounding than the Blues Jr., though not quite as loud. Whereas a Blues Jr. has a very midrangey, in your face tone, the AC10 is more balanced, with sweeter highs and much more present lows. I actually like the digital reverb on the AC10, and use it quite a bit when I play clean or with just a touch of distortion. The AC10 distorts in that great, classic Vox way; don't expect it to do hard rock or metal without some assistance, but for classic rock crunch to clean, this cannot be beat. Yes, the AC15 is louder, and does sound a bit bigger, but I got really tired of hauling around a 50 pound amp, and the AC10 is lightweight. No problem keeping up with a drummer, though it won't do pristine clean and keep up with a heavy hitter. If you're thinking about picking up an AC10, I strongly encourage you to spend the extra 50 bucks to get the upgraded speaker. It's more than worth it. Thanks again, Sweetwater.