OK...first of all let me say I'm not necessarily a vintage kinda guy. Sometimes things that are old aren't vintage, they're just...well...old. However I'd never tried an alnico speaker in any of my amps or cabs and I found a pretty darn good deal on a 1963 Rola 12" 16 ohm speaker from a Hammond organ. So I did some research and found that the reviews on organ speakers for guitars were a little mixed but on average experiences were positive taking into account that tone is totally subjective. Basically as long as it wasn't overpowered with wattage it would sound good and last for awhile.
So I pulled the trigger and got it. I installed it in my homemade 1X12 extension cab and hooked it up to my EL84 powered 15W Crate VC2110R. I've played it for a few hours now at various volume and gain levels and I have to say I'm pretty darn impressed. Maybe it's just the fact that it's alnico but it's so much smoother than either the G12H30 or the G12M25 that I've been using in that cab. It has a really creamy sound on the neck pickup and when you add some more gain on the bridge it just breaks up so nice. I've run it much louder than than normal practice levels but I haven't run it flat out with everything dimed because that's not the way I play and it seems to hold up very well. At very high gain it gets a little muddy on full chords but so did my Celestions.
I know there is at least one amp builder here that doesn't recommend these for guitar amps even the low powered ones but I'm pretty happy so far. I'd never run it with my 30W Mesa but for my little Crate it's a winner!