I have a Marshall MG15R with reverb for practice but it always lacked a bit 'ooomph'. I saw this speaker (8", 15W) and have had Celestions in previous Marshall amps and loved the tightness.
Guitars used: Fender Std Strat, Telecaster, Epi LP and Fender Jaguar.
Styles: Blues, Rock, funk-rock, Heavy metal and even a bit of Jazz sometimes (just about anything!)
And that is what it does, it makes the Amp sound tighter and better pronounced... The lows are deeper and higher notes have more detail. Overdriven it produces a nice roar, that is just a little bit more like a higher end Marshall.. It makes a small amp sound a bit more mature.
Installing it takes less than 15 mins and it's the best upgrade that this kind of money will buy! For the MG15R it was a direct replacement (I didn't have to pre-drill any holes for screw routing
So, bottom line is - If you have a 15W amp with an 8" speaker that sounds a bit tired? - I can really recommend this!