I did the same thing with Wool. I bought it and it sat there unread for the longest time. One day I got the urge and I flew through it. Loved it. I bought the Shift Omnibus and then...well, I let it sit.
I like variety. I might like an author a lot, but I still won't read his or her work more than once a year. Hugh Howey is definitely becoming an exception to that. I've already read Wool and I, Zombie this year, and now I'm reading Shift. We'll see how long it takes me to get around to reading Dust.
Now on to my review (I don't think there are really spoilers here, but proceed with caution). As with Wool, it took me a chapter or so to get my bearings. Once I got into it, I thought it was practically perfect. Wool #1 knocked my socks off. It was like nothing I had ever read before, and had a huge impact. I have a similar feeling about First Shift: Legacy. It was a little different because I knew basically where it was going. That didn't lessen the impact, however. I loved how the details fell into place and watching the silos develop. I love knowing how everyone got into the silos and who is in Silo 1. I loved reading about what happened in Silo 12. There are still a lot more questions to answer, and I'm looking forward to reading the next part.
I'm starting Second Shift: Order, but I also need to finish reading my book club's August selection (Sutton, by J.R. Moehringer). That's good, too, but I'd rather be reading Shift.