We stayed 2 nights at the Juniper Springs Resort at the beginning of June 2012 as a stop-off between Yosemite and Vegas. Originally we were going to just stay 1 night but decided to book 2 so that we could explore the area a bit and have more time to rest between 2 long drives (5 hours from LA to Yosemite, 6 from Mammoth to Vegas).
Initial impressions of the hotel were mixed: on the one hand it looks pretty dated but I guess in the ski season with snow around it would probably look "cosy", on the other hand the reception service at check in was very friendly (a cynic might say it was just because they aren't busy outside of winter season so weren't rushed) and we were offered plenty of advice on places to go and how to get around.
The room was okay, pretty large and full of amenities we didn't really need (or use) such as a cooker, fridge, sink etc,) but again if you wanted it, it was there. Unfortunately we slept terribly both nights which I don't think is entirely the hotel's fault (I suspect the 9,000ft altitude didn't help) but the room was quite hot and stuffy and, like others have said, there is NO air-conditioning here, just heating and fires for those winter times. (Definitely aimed a ski tourists not summer visitors).
As for Mammoth itself, it's a pretty little village and easy to get around, a typical ski resort I would say. There are a few shops and restaurants so for a 2 night stay we had plenty of choice for eating out, though I suspect anything more than a week you'd struggle to eat at different places every night (unless you wanted fast food or takeaway).
We were a bit gutted that access Devil's Postpile was still closed when we visited due to the snow - being the end of may we assumed it would be open but I guess that's our fault for not researching properly. However we did take a gondola ride up to the top of Mammoth Mountain which is worth doing - at 11,200ft it's pretty high and worth seeing if the weather's clear for some excellent panoramic views of pretty much the entire surrounding ranges.
Other than that we just wandered around, went on a hike to Ruby Lake which is very much 'off the beaten track' but worth it if you're prepared to hike for a few hours, the views are stunning.
So would I recommend Juniper Springs? For skiers it's probably ideal in the winter for it's location and well equipped rooms. In the summer it's okay for a stopover but I'm not sure what else you would find to do in Mammoth. There are probably cheaper hotels in the area and if you aren't picky then Juniper is probably expensive for what you get, but we were too scared of ending up somewhere terrible so paid a bit extra and weren't disappointed with what we got. Also the hotel is right on the back edge of the village set slightly away from everything else, so if you don't have a car (which seems unlikely especially if you're a summer visitor) be aware that you'd struggle to walk to the shops/restaurants from here - I guess it'd take at least 30 minutes.