What should I look for when shopping for an ESD bench mat?
ESD precautions:


  I would like to get an anti-static mat for my bench top


Buy the cheapest one in the size you need.

Suitable materials for ESD desktop sheets: Any somewhat conductive sheet of material that is grounded via a 1 to 10 megohm resistor will work as an anti electrostatic work surface.
 (Sheets with very low resistance per square risk conducting current when PCBs etc with exposed terminals are placed on them. (Ask me how I know :-) ).

Sheets need not be more than a trace conductive, although very conductive sheets will work. eg a thin sheet of galvanised steel would work well but be immensely inconvenient and potentially very dangerous as it allows instant conduction from anywhere on a work bench to anywhere else.

Better cheaper faster ... A potentially much cheaper than "real" solutions product is Butyl Rubber sheet sold for roofing and pool liner purposes. The conductivity in this material is caused by carbon-black which is integrated in the rubber and it is wise to check that at least some conductivity is present. Take an ohm meter set to say 10 megohm range with 2 sharp probes. Press probes into sheet just not touching so that probes penetrate sheet surface. If there is even a trace of resistance measurable then the sheet will work as an anti ESD sheet. Results may vary from megohms to a kilohm or so depending on carbon black loading. 

Measured resistances: Notionally the "resistance per square" of a sheet of conductive material is the same at all scales so the resistance across say the diagonal of a square of material should be the same for a square of any length side. So if a 1cm side square measures 100k across the diagonal then so too should squares of 10cm or 100cm or 1m length sides (given the same thickness in each case). In practice results may vary somewhat, but this is a good enough gyide for ESD purposes - if you can see a reading in the megohms of less range at any distance (small or great) the sheet will probably work OK. 

Wood: An older worn wooden desk will probably be an adequate ESD protective surface in its own right. A new coat of varnish may destroy this. An older wooden floored building will also probably be ESD safe. A new concrete floored building will not be and linoleum or carpet add their own potential problems. 

Carpet: Some carpets generate enough "static electricity" to cause user shocks from sparks generated. Your electronics will die even when you just wave your hand near them if not well protected. Use of an antistatic spray on the carpet may stop users receiving shocks but induced voltages may still destroy ICs. 

Especially sensitive devices include: LEDs, Schottky diodes, GaAs devices, MOSFET gates, unprotected CMOS, Gunn diodes (who?). Many modern devices have ESD protection built in - especially those which are especially sensitive if unprotected, but suitable care should stoll be taken. I have seen less sobvious ESD damage over decades than the salessmen would have led me to expect BUT I have seen certain ESD failures and in some cases could virtually guarantee device failure by following ceryain unsafe procedures.

Safety: Mats and wrist straps should have a high value resistor - usually 1 megohm but not critical - to avoid grounding users for power voltages and so creating a shock hazard.