I always try to build my own, but most of the time ends up in a dissapointment without some guidance. After reading / trying some tactics i blend it into my own view of things.
So i don't literally use someone else's tactics, but do use it to get a better understanding of the AI / view of FM.
I mean, with the old ones it was possible to create one youself. But, especially since 2007, it didn't feel any longer like i was building a tactic, but was trying to crack the AI. But i have to admit, 2008 looks pretty solid and above all more logical. Changes i make which i notify in a match don't longer get punished because of some strange behavior of the AI but actually do make more sense and do help.
So yes, i prefer to build my own one. As long it stays possible within the period i can handle it. When my mouse gets trown to a wall again because of the frustration of why the players are not doing what i tell them to do (at least in the big picture) my motivation drops to borderline behavior and believe me makes the game way more expensive then it needs to be
Anyway, i do like this engine and am building a pretty solid tactic for my team at the moment which works overall very well.
I did downloaded some tactics, just to see if it was possible with 2008 or it was just a big pain like with 2007 though. Once i saw it was possible i was happy to build my own