Never used the 990, but I have a pair of 992s and really like them. They're a little bright, as mentioned earlier and yes, they do accentuate sibilants and plosives when used for vocals. They're a bit on the bright side, and it's easy to get a harsh sound if not placed properly.
But overall, for the cost of an SM57, IMHO you get a little more than you think you're paying for.
I really like them on acoustic guitar, room mics for drums, and with the proper placement they can sound pretty good on violin. I've used them as a stereo pair for recording choirs, and the results were pretty good. I really like the way they sound micing my old Ampeg Portaflex, seems to really capture that Ampeg 'growl.'
Some uses where they have been problematic for me have been banjo, dobro, and female vocals. It's really difficult to capture those sources without them sounding harsh or tinny, but then again capturing a dobro or banjo without excessive tinniness can be a challenge within itself...