I've bought at Mountain States, Groove and Autonation all within the past 5 years.
All were fair to work with, however Mountain States was the hardest of the 4 to negotiate with. Not that I'd stay away from them solely for that reason, because I think their price structure is fair, but I feel like I paid the least at Groove, and Autonation was a close second.
As for overall process, groove was absolutely the smoothest I've dealt with in awhile. Really easy to get through finance and they did a few "we owe" items while I waited. If you shop there, Kevin Gaw is the internet manager, and a great guy to deal with.
Autonation was fairly easy and straightforward and treated me respectfully as well.
I did not like the finance manager at Mountain States. He offered to buy down my APR if I purchased extra warranty items/windshield protection, etc. I told him no, but I'd like the lower rate, which he declined. I'm not in the industry, so I could be missing something but I don't think that me purchasing warranty products magically gives the finance manager the ability to buy down the rate. I almost walked because it felt like a shady deal.
Hope this helps you! I buy cars often so I have a lot of dealership experience, feel free to PM me if you have any specific questions that you'd like my opinion on.