I loved the first Resistance. Very tight controls, fast action, and an interesting plot (to me at least). I liked that the weapons didn't get too fantastical; the Carbine is my bread-and-butter, but the Bullseye's tag ability is also very handy (especially when playing against a Chimera team in multiplayer). After playing Resistance regularly for months, Halo 3's multiplayer felt like walking through molasses; I couldn't stand playing it. Resistance has a frantic pace (it reminds me of the older TimeSplitters games, actually) and your typical deathmatch is over in less than 10 minutes (especially when you have a couple of good players in a game).
My main complaints about its single-player is that all of the environments start to look the same near the end. The ending is kinda anti-climactic as well. They addressed some of this in R2; R2 is way more colorful and has much more varied locations. It annoys me that R2 doesn't have a narrator like R1 does to tell you the story; you witness Hale talking to other people and the story unfolds "in front of you", so to speak. My ideal would be the story unfolding in real-time alongside a narrator, but I'm digressing. I loved R1 and R2 is very good as well. The Resistance games are highly underrated in my opinion, but it's easy to get lost in a genre that contains giants like the Halo and Gears franchises.