I have had really bad luck on Rainier: 2 unsuccessful attempts- one bad weather, and one sickness in the party (Liberty Ridge & the cattle route, respectively,) and about 5 weathered-out planned attempts.
I don’t climb anymore, BTW, but when I come back to the sport, Libery Ridge is top 10 on the list. Rainier is a really, really beautiful peak, and well worth climbing.
It’s a big mountain, and if you are climbing the cattle route, (and in reasonable shape,) summitting is a bit of a crap shoot; in good weather you can climb that thing in your tennis shoes. In bad weather, you ain’t getting up, period. For me, this was frustrating. I had all the right “stuff” to climb it, but the weather gods hated me. I had better luck climbing in Alaska, fer chrissakes…
As Tom suggested, Rainier is a textbook fast and light mountain, because you just aren’t getting up it if the weather is bad, and it’s only a 2 day climb. You won’t find yourself bivying while you wait for a window to make the summit, so you essentially climb with a dayclimb/disaster pack. (I’m assuming you will go up through Camp Muir.) Day 1 to Muir is nothing but hiking, and you go down if things look iffy.
Gear for the climb is pretty easy, and previous posters pretty much nailed it. Make sure you have spent a large quantity of time in your boots & pack (at the weight you will be carrying.) I always carry more sleeping bag then my partners because I sleep cold - you should try and figure out how you sleep, and bring the right bag.
If you wear contact lenses, and haven’t climbed at altitude with them, make sure you have a backup plan (like, well, glasses.) Some people just can’t keep lenses in when they get up high, and it is a serious nightmare.
Make sure you are up on glacier travel and rescue skills. A climbing buddy of mine went waaay deep in a crevasse on Rainier, and it would have been super-epic if he hadn’t known what he was doing.
If you are going the guided climb route, I would second the recommendation of North Cascades Mountain guides. Steve House is an old friend and mtn. bike buddy, as well as a great guide and the ultimate climbing stud. He is also one of the most experienced guides in North America, which many people do not know. He’s not just the guy who scared Dr.Doom, he’s the guy who was the youngest certified Mtn. Guide ever to come out of this country.
…Of course, RMI is great, too, and I have known/climbed with a bunch of their guides over the years, and all have been extremely good at what they do.
MH