Does acoustic insulation mats work on pc's?

Well, yes and no. It looks like it's just a foam sheet, which can be bought elsewhere cheaper.

Materials such as Dynamat would be far more effective in standard sound deadening applications compared to foam. And if you want a cheap alternative to Dynamat, you may consider roofing shingle tar, which is the same material.

But, it depends where the noise in your PC is coming from. If it's all coming from air blowing through filters and fan grilles, deadening the case won't help much.

Hmm pros = some sound absorbing
cons = can increase case temperature.

I would look to improve your cooling since fans are where most PC noise comes from.

Your fans will more than likely pe screwed directly to the case panels causing vibration to resonate - you can cure this :-

1) buy quieter fans or slow your fans down
2) use rubber grommets / washers to try and reduce the effect of the vibration
3) change your pc case.

I have in the past used quieter fans (havent we all ?) but for a couple of pounds you can buy rubber grommets / washers that can be very effective.

As far as adding insulation to a PC there are three main considerations:

1. Is it fireproof? I'd be very cautious about what I put into a computer case because if for example a fan wire gets chewed by a fan then sparks can be produced. I've seem some so-called computer insulation kits that were extremely flammable and therefore dangerous. Not only could they easily catch file the fumes were highly noxious. Use only tested and approved materials.

2. Is the foam designed to be used for sound absorbtion or is it simply generic foam? Properly designed and manufactured acoustic foam is open-celled and efficient at being acoustically absorbtive. Generic foam is closed-cell which means sound simply bounces of it and ricochets around inside the computer case. It may add a small degree of insulation but it is will not be very efficient and generally a waste of money.

3. The best acoustic foam products for stopping sound from getting outside of the case have multiple layers of material and these layers have different densities. One layer will absorb sound and another layer (usually a very thick and dense layer) will stop mechanical transmission of the sound and prevent the case itself from being an acoustic amplifier.

The best product I've ever seen or used is made by Acousti Products and you can buy it from Quiet PC USA here:

http://www.quietpcusa.com