I've been to Rosita's quite a few times now and it's consistently pretty good. Salsa gets brought to your table within a minute of being seated and tastes great. The last time I got the fajita quesadilla and their sangria margarita (special of the day) and both were excellent!! Especially the sangria margarita. It really hit the spot during the warm evenings of Tempe.
As far as the staff goes, they're very friendly most of the time. It was only once that my server seemed very annoyed to have to serve tables. I haven't seen them since.
On the downside, I do not recommend the guacamole. It's similar to an avocado paste or puree, which is a bummer. It could be as delicious as everything else and is significantly worse. I had to mix in some salsa to enjoy it at all.
All in all, I think it's a good Mexican place that's unassuming, friendly, and quiet. Nothing too special, but I don't think you'll be disappointed.