Don't like them too much, except Crossover. My problems with the DS9 MU are:
1. So most of the good guys in the normal universe are...good guys in the MU?
What a wasted opportunity. It could have been much more "mirrored" than it was. Evil prophets, liberator changelings, Sisko as an evil Emissary dictator sending his Bajoran troops to invade a peaceful Cardassia, whatever.
That's why I liked Enterprise's MU much more. It was great seeing all of them as evil scheming assholes. Sisko as a poor man's Han Solo? Not so much.
On the other hand, it's clear why they did it. DS9's setting was already pretty grim, so they thought making the MU more comedic and light would work.
2. As mentioned the "let's make it so we can reuse all the sets we have" approach. DS9 is the center of this whole conflict? how convenient! They also build a Defiant replica, they have the Maquis raiders, and UM Terok Nor looks exactly like the "real" Terok Nor (IAMD had the same problem) ? Interestingly, the DS9 MU episodes have some very good FX. I'd rather have them use the money on an alternate Terok Nor or at least SOME new costumes, vessels and sets.
I would have liked to see a little less from the DS9 corridors.
That's where DS9 was often underwhelming. In every Celestial Temple ep after the pilot, the prophets "are" the dudes on the DS9 ops, when Miles and Bashir get in Sloane's mind, it's the DS9 corridors, when Joran haunts Dax, she gets visions of the Defiant corridors etc.