Should I take the plunge?
I would say yes--it is wildly uneven ranging from brilliant to awful. I thought S1, portions of S2 and S4 were the better years for the series. S3 and S5 were pretty average and disappointing.
The show really doesn't break any new ground for the genre--with SN you shouldn't expect any new spins on old chestnuts. Plotting isn't its real strong suit so I find the arcs of the season rather disappointing--serving mainly as a MacGuffin in the background--although I think one reason why I enjoyed S4 so much was how well it balanced character and plot that year.
I got into Supernatural since I really like supernatural/horror shows. Some of my favorites are Tales from the Darkside, The Outer Limits, The Twilight Xone, the late 80s cult fave Friday the 13th The Series, The X-Files. The show does a fairly good job with its atmosphere and make-up despite a limited budget in the middle years.
I'm 33 and I do at times feel like the show caters a little too much to the teenager crowd and one aspect I really can't stand is the propensity of the show in recent years to do a lot of comedy episodes that are low brow and filled with all sorts of juvenile humor--I prefer serious dramas to not tread into this arena that is why I loathe True Blood, Buffy, Angel, Hercules, Xena, Lexx, Dr Who.
Sff these days pretty much suck--I can't stand any of the SyFy channel shows like Haven, Eureka, Warehouse 13, CAPRICA.
I'm a fan of TNG, DS9 and the third and fourth seasons of ENT. Of the newer shows in recent years I liked LOST although the weak way it ended was aggravating; nBSG was good for the first season and half of the second; loved the first season of Heroes; The Vampire Diaries I find to be one of the better shows these days. The Event is also picking up. I enjoy Chuck, unapologetically admit to considering myself a fan of Gossip Girl. I also enjoyed the original Melrose Place, DALLAS, and Hill Street Blues. Can't stand all the crime procedurals like CSIs, Law & Order SVU. I could appreciate the Sopranos as good drama it just wasn't my cup of tea.
Overall I say go for it. It isn't as good as The X-Files and it can't hold a candle to a show like Friday the 13th The Series which embraced its dark nature but in the sea of crap these days it should mostly satisfy.