Rainier is a darned good test of a boot. The biggish pack, the long uphill slog on day 1, all the twisting and torquing when wearing crampons on day 2, the brutal downhill. Oh, the downhill. That last two miles down the Park Services stone steps, Paradise Lodge easily in sight but still so far away, are just miserable. My knees ache just thinking about it.
A lot of people wear plastics on Rainier but unless youre making a winter climb I think thats overkill. Better are the newer generation of plastic/leather hybrids, or leather boots. More comfort, good weather protection, excellent hiking and climbing performance.
A really good example of that is the Asolo Expert GV mountaineering boot ($300). It has uppers made from waterproof leather and tough synthetic fabric; a Gore-Tex liner; a supportive but flexible midsole; and complete crampon compatibility. Theyll feel good on the lower half of the mountain, with enough warmth and climbing performance for the top half.
I also like the La Sportiva Karakorums ($270). These are a bit more traditional, with uppers made from 2.8mm Perwanger leather, which is tough and waterproof. Rubber toe bumpers protect your feet, and snug cuffs keep Rainers nasty scree out of the boot tops. And theyll take most crampons out there.
My third recommendation is the Scarpa Chramoz GTX ($290). These are all-synthetic, with uppers of nylon and synthetic nylon. That helps make them exceptionally light (just over three pounds), and theyll dry fast when melting snow wets them a bit. Gore-Tex liner, Vibram solesall the good stuff. An excellent mixed-terrain boot. Wont even require much break-in.
Have a great trip! Let us know how you do.