It's very nice. Nicer than most, definitely.
A pet peeve I have with mission sites in general is that their front pages tend to make it hard to tell what you can do with them. (We used to call this "hiding the chalupa"). Web users focus in on the primary area of a page almost subconsciously. This is the area roughly 400x300 a little down and to the right of the top right corner. Unless end users know already what the site does, or unless it follows a recognizable structure like a board or a search engine, they expect that area to contain a link to what this site does, or a menu or description of the same. If it's not there, they get confused.
The first page hit, without scrolling, also needs to convey a rough site structure for mental note, and an overview of what's possible for a variety of user types.
Mission sites tend not to do this well. HiRISE's first page hit is a decent example of this. On the first page hit, without scrolling, it's hard to tell what the primary use cases of the site are
as an end-user. "What's an anaglyph?" (I know what it is, but I'm saying an end user wouldn't.) "What's a stereo pair?" "What's on this site that I can understand and use?" It's possible the HiRISE website people did an analysis of their user base and decided to focus in on the more technically apt enthusiasts and the scientists using the site. But even so, the support for casual enthusiasts and plain curious-but-not-enthusiastic taxpayers still needs to be there. Typically your more advanced users will find what they're looking for no matter where you put it on the front page.
Also, as a general rule, you don't want a menu with more than 7 items in it. People can navigate & remember 7 items with their short spatial term memory. 8, not so much. 9, forget it. A 9+ item menu says to the first-time user, subconsciously, "ignore this for now".
The end effect is that the site is off-putting to the casual users that help pay for it.
All that said, HiRISE does several things much better than most. It does the front-page hit decently well, once the user overcomes that first critical initial impression. The big, clear, web-sized pictures are great. It reminds me a bit of boston.com's
The Big Picture (which, BTW, is a great example of how to structure a site simply and clearly and utilize the web's basic strengths - note in particular the great use of vertical scrolling, something all browsers do well). I want more of those web-sized pictures in general. The layout is nice, wide, crisp, and consistent. The fonts are easy to read and in good contrast. The pages aren't noisy with a variety of small images and table structures.
In general, I'd recommend concentrating more on the 4 or 5 primary use cases the site provides, particularly the 2 or 3 all users could care about, putting the 5-10 secondary use cases "off to the side" a bit more, creating a (secondary?) blog-like structure for news so there's a visual temporal component, and presenting more medium-to-large sized images on sub-pages (don't be afraid) with links to larger blow ups and other formats. And all the great stuff HiRISE is already doing that I listed above.
Phew. Sorry for the long post.
Guess I care about this stuff more than I thought!