Actually, you don't hear Mordaunt Short mentioned here at all......or very rarely. I see one member has some 900 series speakers in his signature.
I owned the 500 series that were released a few years back. The 502 fronts had built in subs on each speaker. The 504 center and 506 surrounds filled out the 5.2 setup. Retail on that set up was around $1800. I bought them used. I can't speak for the models you are looking at, but the 500 series was an excellent performer in the mid fi speaker class. The fit, finish, and heft of the units was spot on for the price. The sound quality was excellent. Some feel they might be lacking in the low end as a brand but I am all about the mid and upper ranges so they sounded wonderful to me.
At that price you are getting a decent speaker a couple steps up from entry level home theatre. In the future you could upgrade to better fronts and still keep the other speakers in the mix.
The company is an authorized dealer. They have a thirty day return policy, with you paying for shipping if you don't like the speakers. HTH.