I have the SX, which, if I am not mistaken, is effectively the 4-channel version of the SR.
The SR is a 4 DECK controller (hence the decks switch buttons), but only a 2 CHANNEL mixer part. This means that your signal for deck 1/3 comes in on the same fader, making it hard(er) to mix. You can’t really mix from channel 1 to 3 really, but you could mix from 1 to 2 then from 2 to 3 and from 3 to 4 if you like. But using more than 2 decks simultaneously is difficult as you can’t easily control all 4 deck volumes.
My SX seems like a reliable piece of kit, although I have only owned it a short while and haven’t used it very often (played out a few times and practiced some). I would have no problems taking it to gigs, but that is just my opinion.
I think I have heard mostly positive news from the people on here that own one. But a quick search on DDJ-SR should give you some more info on that.
Hope that helps a little.