It is a tourist trap and often not worth bothering with it. My philosophy on it is mainly "look but don't buy, IF its not too crowded".
I only ever find myself stopping there in the late fall/winter/early spring. Thats sort of the off season. Avoid that entire exit/town like the plague in the summer time, good lord, I made the mistake of attempting to stop there with a friend who had never been in July and it was the definition of full retard. They have a pretty big lot and there was not a space to be found. From the time we looked at eachother and said "F this" it still took 30 min to get back on 95N due to the traffic. Just a sea of MA plates, yuppies from the city going to hang out in York or Oqonquit.
I'm into almost every type of outdoor/firearms related gear imaginable so I can still easily kill an hour + looking at stuff. The volume of firearms can be interesting, I like to browse the long gun racks to see whats around and if you want to fondle a specific gun, its usually there. Ammo is hit or miss but always pricey, never walked out of there with anything close to a "score". Id say the place is mostly an avoid but can be interesting if not crowded.
As for Shooters Outpost, its hard to argue with their stock of ammo and ammo prices. They are a little too into enforcing MA law, do a search for that can of worms, which irks me but not enough to totally avoid. The gun people upstairs are generally pretty helpful and will let you look at whatever you want. The feel is a bit polished, the long gun racks have everything locked in place so no real fondling as compared to Rileys up the street. I still get the overly suspicious feel in Shooters from many of their staff or more likely their management. I want to like Rileys more but while the ammo supply seemed to run about neck and neck shortly after Newtown, Shooters has far and away pulled into the lead for regularly having what I want in stock. No contest, the past 5 or so times Ive stopped looking for ammo at Rileys (I go up there alot) ive left saying "why did I bother?". Shooters is the opposite 98% of the time I leave feeling like I got a good deal.