Iskaryot, as you can see by the numerous players that have responded to your quiry, a pistol is infact very effective on the field, depending on how you utilize it.
They are effective as a back up "weapon" incase your primary fails on the field,
They are a great substitute as a primary for CQB and even better for those fast/stealth type players,
They can indeed hold their own against an AEG given the right situation (though don't get into a pissing contest against an M249),
And.......they are damn fun (they are the candy of airsoft).