I think thats a tricky question, personally, depending on how you want to define 'worth it'.
Competitively, they won't be worth it very often. There are exceptions, of course. da gold sun mentioned CB Heatran. A while back I actually used something of the sort in the form of a 'mixed' set. Ran Iron Head, Earthquake, Explosion, and Overheat. It actually got a decent amount of KO's, surprising common switchins like Blissey with what could often be a 2HKO in the form of Iron Head. Then, later, when they realize what you're running and bring in the likes of Skarmory it can eat an Overheat instead. Explosion, needless to say, hurts like a bitch and has brought down beasts like Hippowdon and Cresselia. Though most of the fame on that team went to a CounterWishBliss with Toxic Spikes support. That thing got more KO's than any Blissey should ever get and once caused a guy to forfeit after losing his CBHeracross, AgiliGross, and Specs Alakazam to it. To be fair the Heracross had been weakened to a point where Protect killed it beforehand courtesy of poison.
While competitively not often worth it, gimmick sets can often be VERY fun to use and thus worth it in that sense. A recent team I used, in fact, consisted of (and I'm not joking) Luvdisc, Pikachu, Phione, Shuckle, Bastiodon, and Regigigas. Needless to say, it didn't win often. In fact, it won a whole one match. Had a few close ones though like when it was down to my Luvdisc and Regigigas against a Garchomp and Skarmory. Spikes/Stealth Rocks did me in eventually but it'd have had to be the most fun I've ever had losing. Pikachu claimed the lives of several Breloom who, for reasons unknown to me, thought that they could outspeed and sleep me. The players must have existed in a world where 70 is greater than 90...