Are they any good? Very biased pinion here, but I would say they arent good for SPL or SQ - the are sh*t. period.
For SPL., heres an example, although these subs are not as popular as they used to be, possibly cos of products like the DDs. The shocker xtremes are just the tip of the Shocker Audio iceberg... they will handle all of 2500w rms per voice coil...and more.... they are loud did you know that 7 out of 12 db drag records broken in 2001 were with these subs.... I guarantee that I could take you anytime any day with one at xtreme 12" against at least 4 8000 12's.... and you can quote me on it... (oh and shocker sig series 12 quad coil.... 5000wrms per coil 4 inches throw.... bass that will kill you!!!!!)
other subs that will rock your world like a shocker...
Hurricane Powerstroke
Crossfire BMF
Juggernauts
Treo CSX
Beyond audio- inhuman, PSi
Resonant Engineering-12hc
Image Dynamics- IDMAX
Audiobahn-1205x
Cerwin stroker 12
Memphis SHP (held doors 4van with 4SHPs - did over 168dB, with4 SHPs and 1500RMS to each one)
AT shocker II
DD 3512, 9012, 9512
Orion HCCA
these will spank anything your mtx could throw and not even break a sweat... come on oh yeah DD9515 holds record for single loudest speaker... 170.8 db from 1 15" certified... on a TERMLAB MIC.....
For SQ....
For SQ you have:
ID IDQ
ID Max
Eclipse DVC
OZ Audio
Crystal CMP
Kove
these are the only names you need to know......oh and Beyond Audio too!!!! (SPL AND SQ - check out their website)