I don't think any of the precons are "playable" in the form that they're presented in. They're supposed to give players a general feel for certain aspects of the sets and really lack the cohesiveness of a real constructed deck.
But that doesn't mean I don't like them. I think most of the precons provide an excellent foundation for starting a deck. When Mirrodin was released, I was able to build a pretty decent affinity deck using the Sacrificial Bam and Bait and Bludgeon precons. Of course, as time went on and everyone realized how over-powered affinity was, I couldn't play it anymore, but it's still a good example of the usefulness of precons.
The second reason I like precons is so I can get specific cards I'm looking for. I used to get booster boxes and sometimes I'd only get 3 of some uncommons and use a precon to round it out. In Fifth Dawn, I bought a Stampede precon in order to get my fourth Eternal Witness and Magma Jet online instead of getting ripped off trying to trade for them. I did the same thing in Ravnica, buying the Golgari precon, which came with 2 Putrefy, 2 Darkblast, 2 Recollect, a Moldervine Cloak, Savra and Golgari Grave-Troll - all cards that I wanted. And, of course, everyone knows that the Betrayers' Rats Nest precon comes with Umezawa's Jitte and used to be able to get purchased online cheaper than the Jitte.
So anyway, if you want to buy a precon, look at the rares and uncommons. If they're cards that you really want and you don't think you can get them for cheaper, then go for it. Personally, none of the precons in Guildpact or Dissension really impressed me much. But I certainly think they're a good tool and a good deal for the right people.