|John's post raises a couple of points worth mentioning that I think make the HP compare favorably:
1. Cartridges -- HP's earlier foray into multiple color cartridges, the 02 ink system -- required colors such as light cyan, light magenta, etc. The new 564 ink system used in the Photosmart Premium has dual-drop technology -- i.e., precision color mixing by using a combination of a large and small drop sizes. To compare apples-to-apples, this technology makes the new five-ink printer function like an eight-ink printer -- all three color cartridges have the dual-drop design.
2. Dyes and pigments -- The HP five-ink printers use both. The photo black cartridge is pigment, the three colors and text black are dyes (see http://www.hp.com/hpinfo/newsroom/press_kits/2008/printingpro/bg_564.pdf for details).
3. Price -- The Photosmart Premium is typically $199 U.S. unless on sale and does quality work on media up to at least 8½ by 11.
I haven't looked in detail at Canons recently, but historically my perception is that Epsons are the next best option after the HPs when making valid comparisons at a given price point.
Regarding John's comment on moisture resistance, bear in mind that the print medium plays a huge role here in both the quality and longevity of the print.
Hope this helps, too,