Turntable Mat question

I'll go with millercarbon on this one. Bad idea unless all your records are 200  grams. The best mat I ever used was glass plate with a thin layer of felt on top. It had a recessed center and the diameter was slightly less then a record. Dead as a door nail. SME,  Kuzma, Walker, SOTA, Basis and the Air force tables use the best record hold down systems. SOTA was the first to use vacuum. SME, Kuzma and Walker flex the record into the mat. 

Those platers were specifically designed to use w/o a mat but to couple the record appropriately to the plater you have to use a clamp and a ring
(for another $1000). The ring is a PIA. SOTA, SME, Kuzma and Walker have much more elegant solutions to that problem. There is no such thing as a flat record although properly pressed 200 gm records come close. With any pivoted arm you are going to get warp wow unless you flatten out the warp. That is something you can actually hear. And, an appropriate mat should sound like absolutely nothing. All good mats should sound exactly the same unless you have a problem with your platter ringing. Tap to find out!

Nobody here heard a turntable with vacuum hold down? Basis, SOTA and Air Force take things to a whole new level. Walker also. 

Atmasphere, I have been using vacuum hold down for 30 years and have never damaged a record. I suppose if you threw sand on the platter before the record you might be able to do some damage. No flat mat of any material can control a record that is not flat. They can dampen platter resonance and to some extent in the record (fortunately vinyl is pretty lifeless stuff) A properly designed vacuum system works perfectly all the time and every time regardless of the condition of the record at some additional expense and another box ( big box for the Walker) 

Rocky what Ralph is saying  is that the center of the mat should be recessed to fit the elevated label area and have a total diameter slightly less than the record so that the fatter record lip can hang over the edge of the mat. This way the playing surface of the record is in fill contact and supported by the mat assuming the record is flat. Any mat or platter that does not do this should be avoided. Now no record is perfectly flat.
There are three systems to press there are three systems to press the record into the mat forcing it flat. All systems use a center record clamp. The there is the periphery ring ala VPI, Clear audio. Vacuum hold down ala SOTA, Basis, Air Force and finally the SME, Walker, Kuzma solution which is a bit harder to describe. There is a thin washer about 1" in diameter placed over the spindle under the mat. The record clamp is designed so that it only contacts the record label at  the clamps very periphery. So, now you have the record floating in the air the thickness of the washer so that when you screw the clamp down it flexes and forces the record into the mat. If you try this with an old 78 (shellac) it will crack and they warn you not to do this but vinyl is suitably flexible to take this.
My own opinion is that the periphery ring is a PIA. The SME clamp method works but stresses the record and I have seen 200 gm records crack a little at the center. 180 and below no problem. I personally like the vacuum method best assuming it is designed correctly using low vacuum pressure and a noiseless pump. It is easy to use and it works brilliantly. I think the bass and dynamic range are improved but I have no evidence to back that up and as always it could be psychological. If you want to think it sounds better it will. 

I'm surprised uberwaltz. The washer that comes with the Mitchell clamp is very thick compared to the other systems I have used. I'm sure it compresses some and I have not use one myself as my turntables are already accommodated. Keep an eye on your 200 gm records. I bet if you look closely right around the spindle hole you will see a few little cracks show up. It does not effect playback at all and I doubt it will ever effect playback over a person's lifetime. So it is a mute issue but, it does happen. The system is also way better than not doing anything and I think better than the periphery ring which is a PIA to use and puts a lot more weight on the bearing which can't be good for it. In the case of the Clearaudio with the magnetic bearing you have to use the ring every play or the bearing will not sag as much changing the VTA and with a pivoted arm the azimuth even though it will not change the wear rate of the bearing.