Shadow Priests are great to play and extremely fun. If you were starting at beggining of season 9 you would be a great addition to any team due to your defensive dispel (aka 2 dispelers in the team) and considering your healing past I reckon you would enjoy it a lot, now things are a bit changed.
I specc-ed shadow beggining of December cause discipline priests were quit shit once 4.0 hit live servers and dont think I am going back to discipline. I would say I am average (2300-2400MMR) but that is due mostly that I play with a new team rather the class or my skills (we overlap some cc).
I play shadowplay (Affliction lock, resto shaman shadow priest) and HeroicRLS (shadow priest, rogue, resto shaman).
I choose the first because of the broken state of Vampiric Touch dispel protection, UA is doing a great job dotting everything and securing that my dot stays there (90% of the cases a healer lands a UA dispel, we switch and destroy him).
I choose the second when i want to have some fast paced games cause apart from DoT damage I have burst on demands and I love poping Dark Archangel with trinkets and shadowdance on my rogue and destroying something in a smoke bombs.
By the 2 statements above what I want to say is that you can have dot pressure and burst on demand so I wouldn;t call the class weak. You can still dispel yourself which is huge vs mages and moonkins, mass dispel your healers cc (watch for ua though), have infinite mana (SW
, Hymn of Hope and Disperse), big save-my-arse cooldown (dispersion), Life grip which can be huge vs TSG and you can either use glyphed fade or the 4/mooncloth set bonus to run to the healer and fear the crucial moment. I prefere the 3/2 as the 400 extra resilience is huge VS cleavers since the defensive dispel los.
I really suck in writting advices out of my head, if you throw a couple of questions it will be easier for me to reply :-)
Hope I convienced you though!