I have not heard that particular model but I have owned 3 pairs of Polk speakers and I've been very happy with all of them - particularly for the price.
Polk is part of Harmon Group and the reason that's noteworthy is because they have extensive speaker engineering, design, and manufacturing capability. Polk is a value brand and the reviews nearly always say they punch way above their weight.
It looks like the S50 has been superseded by a new model which is the reason they are so discounted right now. I would jump all over this deal and get them ordered. This is a flaming good deal and I am very confident that you will be happy with them.
The only thing I would recommend is that once you get them set up you should save up for a subwoofer. It doesn't have to be a big fancy model. It will add the bottom octave and enhance your sound tremendously.
I am running a pair of older Infinity 360 speakers for my TV which have a similar design to the Polks. Like the Polks they got great reviews for such an inexpensive speaker. I added a Mirage 10" subwoofer (Omni S10) which was a huge improvement (I run the Infinities full range). [BTW, I have a full blown system upstairs with big Thiels and Krell gear.] My best audio buddy who has a world class system came to visit and when we watched a concert video he was blown away by how good that modest little system sounded. For TV, movies, and concert videos it does a heck of a job.
There are a lot of subwoofers on the used market for relatively cheap. A Mirage Omni S8 would probably do the job just fine for under 150 bucks. There are tons of used subwoofers on eBay. I would look for something closeby so you can pick it up and avoid the shipping.
My advice is to pull the trigger and don't look back.