In the world of studio monitors there are two schools of thought:
1. Hi-fi - monitors designed to reproduce your sound with as much accuracy and detail as possible.
2. Reference - monitors that may not sound great, but mixes done on them have a tendency to translate well to other systems.
The Yamaha HS50Ms fall firmly into category #2. These monitors were designed and are marketed as a replacement/sucessor to the discontinued NS10s, the classic "if you mix on these it will translate well to anything" monitors.
The HS50Ms do what they are supposed to do, they provide a reference point. They sound similar to NS10s, and like one would imagine can work well in the same setting. They're also reasonably priced.
These monitors lack any low end below about 80hz and sound downright harsh. I found them to be quite fatiguing on long sessions (at responsible volume levels).
Do not buy these as your only set of monitors. If you are looking to invest in one set of monitors embrace school of thought #1 - Hi-fi. Now this does not mean hike up your price range, just look for a set that has better (aka closer to flat) frequency response. See KRK, Event, M Audio, etc. for more options in the HS50M price range. I would only consider buying the HS50Ms as second pair for reference, but even then, if you want the NS10 thing buy some NS10s.
The pair of HS50Ms I owned lasted for about a year before I had had enough and sold them. I got some good use out of them, but if I could go back and do it all over again I would look at other similarly priced options.