They really do work. But depending on the silencer and the firearm is mounted on; it can be rather quiet or still loud enough to damage your hearing if you do not wear ear plugs.
Take a look at this link. The technology of the silencers tested is a few years old, but it will give you an idea of what is available now. http://www.silencertests.com/results.htm
With a 22lr at 117 dB, 223 at 130 dB and a 308 at 138 dB you can not call them quiet. But it is impulse noise and appears to the ear to be much less noisy than say the continuous noise of a chain saw.
With many of the silencers reducing noise by 30 decibels, they are very effective. This is a 1000 times reduction in noise intensity and an 8 times reduction in perceived loudness. The best silencer listed is over 40 dB which is 10000 times less intensity and 16 times less loudness. A 22lr rifle with a silencer will be pleasant to shoot without ear plugs while an ar-15 or 308 bolt gun will be somewhat comfortable as long as they are shot out in the open. When under weather protection or indoors, the sound of a high powered suppressed rifle reflects back to your ear (especially the sonic boom from supersonic ammo) and will cause hearing loss if you are not wearing ear plugs.
While a silencer does not work much better with subsonic ammo, the suppressed weapon as a whole can be 10 dB less noisy due to the much lower powder charge when compared the full power loads
has links to various manufacturers. You can find a forum at http://www.silencertalk.com
and a forum, law summaries and dealer lists at http://www.subguns.com
Double hearing protection (plugs and muffs) will be very effective, but using a silencer also reduces noise pollution and protects the hearing of bystanders who may not be wearing ear plugs. They also reduce recoil and eliminate muzzle flash. They will not reduce accuracy unless they are improperly mounted and cause baffle strikes.
When selecting a silencer, you need to consider cost, durability, size and effectiveness. Dealer and manufacture service are also considerations. Getting a silencer that you can take apart for cleaning is a good idea if you are using 22lr in it as that cartridge really fouls a can fast. Center fire rifle silencers are usually welded together as they do not get filled up with gunk when using jacketed ammo.
You do not say where you live, but if you are in the USA, then you need to live in AL, AR, AK, AZ, CO, CT, FL, GA, ID, IN, KS, KY, LA, ME, MD, MO (with C&R), MS, MT, NE, NV, NH, NM, NC, ND, OH, OK, OR, PA, SC, SD, TN, TX, UT, VA, WA, WV, WI or WY to own one without a license. ATF approval is routine but takes about 4 months now and there is a $200 for each one you buy or make.