I went to Masa a couple of weeks ago and had very good food. I got the braised pork shoulder over polenta with a fig compote on the side. It was an outstanding dish, the meat was perfectly tender and they did a great job with the polenta (I had my doubts when I ordered it). The portion of my dish was great, but another member of my party ordered the salmon and it was on the small side. Still, considering the neighborhood, the prices weren't too bad (about $18 - $25 for mains).
One of the best deals in Boston dining is Masa's tapas menu which is only available in the bar area. They serve 10 tapas for $10 in the bar everynight and on certain nights (Tuesday, Wed.) you can get all 10 for just $5! The tapas are very good, cover a huge area of tastes (seafood, veggie, meat, etc.) and you can't beat the price. Just avoid going on a Thursday, Friday or Saturday unless you get there really early. Space at the bar tables fills up quickly and it can be an exercise in frustration trying to find a space to eat (they won't reserve tables, it's not first come first serve).
I went to Masa three weeks ago and felt the service was awful. We started in the bar area for cocktails and I got ignored for 15 minutes although the bar had only about 8 patrons. When we finally did get service, the three rounds we ordered (of the same sangria margaritas) were all vastly different. On the last round they claimed they were out of salt, we saw them dump the rimmer because they were closing up, and just didn't feel like getting it back out. We ended up only eating off the tapas menu because we were so unimpressed and the food was marginal.
It was really mediocre for a long time. Then last year, they hired youjnd hotshot Ryan Jaronik from Chicago to overhaul the menu, and the food was a million times better. But the last I heard, Jaronik left---some time in the last month or two. Haven't been there since the new(est) guy started.
But one of their bartenders is a tequila expert, and the scene still seems to be hopping. Who knows? Maybe the menu tweaks are still being executed as well as they were in the Jaronik era....