I'm not going to mince words here -- the A plot with Dax and the dimension-shifting alien was an insult to script-writing, common sense, and every single attempt at characterization over the course of the series. Literally nothing about it made sense, from the sudden life-changing decision-making, to any attempt to make the dimensional alien attractive enough to lead to such a decision, to the fact that none of the other primary characters seemed to see that the entire situation was completely ludicrous. Whoever wrote this episode should be ashamed. Whoever greenlit it should be ashamed. People who happened to be standing near the set when it was filmed should be ashamed. It was just hack writing and a hack episode.
I'll give it three stars because of the B-plot, and that's stretching credulity too. The B-plot wasn't bad, per se, but it was also lazily done. It had so much untapped potential for hilarity that the writer never attempted to explore. That one joke at the very end wasn't nearly enough to make the plot worth it, especially when a better writer could have turned the entire sequence into a really funny farce. The idea behind it could have been developed much better than what we got -- too many potential comic scenes were left a straightforward drama and exposition. Quite disappointing.