For a Crackdown junkie like me, would you consider this one a must-have?
It's hard to say. You're a lot more powerful in Crackdown and being able to leap several stories is deeply satisfying. Infamous is a shooting game, deep down, and I think the shooting is really satisfying -- enemies are smart enough, but still fodder to you, plus as I said above the shooting controls are really really good. But that's not necessarily a plus to a crackdown junkie, since Crackdown's shooting was fairly brainless, enemies were really just there to make you look good. As your powers increase, you become increasingly able to take on huge waves of enemies, though, but early on you'll easily be killed by 3-4 enemies, especially if they're sniping you from rooftops.
Climbing's a lot easier than it is in Crackdown, Cole isn't as slippery as your agent -- to be honest, I wasn't super fond of Crackdown's 'oh, I have to jump up a few balconies? let me leap awaaaay, then back! Okay, again!' Leaping from building to building, though, riding on rails, that's all lots of fun, though, and the missions that call for it can be exhilarating. Exploration is via dead drops and shards, which can be found via the minimap.
Infamous is a story-based game, though. There's no New Game+, and once you clear out the maps, you'll have a hard time finding enemies. It's a really well-done story-based game set in an open world. There's no 'alert levels', so you can't just cause mayhem and expect to find trouble. To be honest, I'm not sure Infamous will scratch a Crackdown itch, but it is a ton of fun to play.