I did my REU there last summer, so I don't have a direct review but I'll tell you a few things about the department.
1 - The faculty there are great! They are open to talk to everyone, super nice and helpful! Definitely a big plus.
2 - The PhD students there seem to be pretty happy with it. They have Applied Math coffee hour every Wednesday, so we interacted a lot with the grad students and faculty. They all seem super happy to be there and sounds like they're doing some cool math (if you're into applied stuff). They also just hang out and play games with faculty and stuff (maybe this was because it was the summer?)
3 - Campus is super nice. It's brand new, so although it is small, everything on campus is in great condition (big difference from my home institution where stuff is falling apart everywhere).
4 - Merced itself is less nice. There isn't a lot to join town and everywhere south of the 99 is pretty sketchy. On the bright side, campus is north of town and about 5 miles from the nearest gas station.
5 - Reputation: It seems like its the pattern that this is definitely the bottom of the totem pole, at least by UC standards. Definitely not the most prestigious school, but I don't think it would be bad. My guess is not too rigorous, so I guess it's really up to what you're looking for.
If you got accepted, definitely worth a visit in my department.
PS: Their parking situation totally blows.