Well, my opinion might not help you much, since I also love Transmetropolitan. But then, I also love Hunter S. Thompson's work.

Planetary is not much like Transmetropolitan, though. With Planetary, what I like the most is this sense of mystery. It's hard to describe. Planetary offers plenty of scenarios that open questions, only a few of which are ever answered, and that is kind of the point and appeal of it. It's also heavily intertextual, with a lot of references to other books, stories, comics and real events. It is, in fact, a study of the importance of stories in shaping our world, and our relationship with the stories we tell ourselves and each other. It's also a cool detective story with super heroes. It is many things. It is very sombre compared to the jocular Transmetropolitan.

So yeah, I love Planetary. I suggest maybe reading a few chapters to see if it grabs you. Can't hurt, can it?