Enteric coated aspirin takes a long long time to dissolve, hence is not appropriate for emergency heart attack treatment. Usually they are orange in color and say enteric coated on the label.

I think when you are HOME, using AlkaSeltzer is best. We studied this in biopharmaceutics, and this product is as fast as an IV shot! Make sure you get the ORIGINAL formula..as AlkaSeltzer makes many things now with that name.

I keep AlkaSeltzer around for other reasons, mostly for pain and my PN. (it works well for PN) . I use inexpensive generic from WalMart...and because of the foil packaging I've seen it stay fresh long past the expiration date. So if you want INSTANT aspirin consider keeping this around at home. Of course you have to dissolve it in water, which would be difficult on the go.
But it can be done with a bottle of water. I used to take it with me to work, and if I needed it after a long day on my feet, I'd break the large discs up and throw in my water bottle and drink that. It is best done with only about 6oz of water in the bottle.
WalMart's product is only about $2.00 for 36 doses. Really affordable.

When aspirin is no longer fresh, you can smell the vinegar odor which is the acetyl portion cleaved off the molecule. This happens when air and moisture react with the aspirin chemistry.

There is a new Bayer product out...but I don't think it is as fast as AlkaSeltzer original formula...
http://scienceofpain.com/en/scienceofbayeradvance.php

Chewing up 4 baby aspirins of 81mg each would be about the dose of one regular aspirin.