Singed is pretty good in less organized play. He is a split pushing demon and his ability in a teamfight to contribute is actually very significant if you are not behind (if you are, just keep splitting and have your team turtle for a while until you attract enough attention to get 3 or 4 man ganked. Proceed to press R and spam laugh as you run away while your team pushes midlane/gets drag/kills lonely enemy). If you are ahead, you can completely zone enemy carries off of your team, burn a flash or two, and possibly flip an enemy into your team if they think they can kite/peel you (#!$@ you
). It is very easy to overextend as Singed so be careful.
Anyway. I'm not sure how effective Singed can be in higher tier, organized play, but mains can definitely hold their own in solo queue. A buddy of mine was a Singed main for a while (looks like he was maining Vayne in S5) and had about a 70% winrate with him in Platinum. He told me once that he stopped playing Singed at some point because he would get bored playing and it would cause him to throw games. He sometimes would break it out when he put his tryhard pants on, but otherwise he wouldn't play him, opting for more "technically challenging" or "fun" champions. Anyway. This might not answer your question entirely, but it's one of those things where I can say I knew someone who was able to get pretty far in solo queue playing Singed and Singed has had some success here and there in the pro scene as a niche pick and SSW's Looper played him at World's with great success in Season 4.