AR15s make the ideal varmint rifle in my opinion, atleast for high volume shooting where you're out popping prarrie dogs or squirrels.
The assets to using the AR15 are numerous to a bolt action. Ease of loading, ease of reloading(such as a 20-30 round mag for AR15 as opposed to having to reload after every 5th shot with bolt action), rediculously low recoil being able to spot your hits, faster follow up shots with the AR15 in event of miss or need to change targets quickly, and in most cases an accurized AR15 isn't giving up much if anything to a factory bolt action in terms of accuracy.
Only downside in my opinion is the AR15s can quickly take off in terms of price leaving the bolt guns behind plus the AR15s can get HEAVY. My accurized AR15 built soley for varminting weighs in at 15 pounds unloaded but I love that rifle dearly.