OK, today was push day, where I tried high ISO and other extreme shooting to push the F550 over the edge. I must say that, although the image below is blurred from slow shutter speed, the image itself is within my tolerance for iso3200 noise on a jpeg image. Going to 6400 shows some banding though, so (no surprise) that's a bad idea for a 1/2" sensor! Note that the bizarre mottled sky is in fact city-lit clouds with the first light of dawn behind them - so that's pretty much what the sky looked like at 6:40AM.
Even pushing up the fill light shows no banding on the iso3200 shots I took. I expect banding would show in underexposed images with fill light added - but it doesn't show if exposure is close. Since I don't delibereately underexpose high-iso shots and them boost them for shadow detail, this is not a deal breaker for me. To stop at iso3200 and expose well is perfectly acceptable for a camera that I can pull from my pocket. If I ever feel the need to shoot that way, then I will need to find a different camera; my K200d dSLR isn't ideal for that either.
As to the zoom lens - many other web sites speak about its kaleidoscopic flare, and I'm able to reproduce that. It's not common on shots I took where I expected to produce it, so another not-so-big deal; I have images of twilit fields with lamps and they do not reveal it at reasonable exposure levels. Here it is - but this is cropped and fill light added to make it look its worst!
Here's another lens issue, a shot of the moon that shows some color fringing (especially the cyan on top). This is a huge crop of a bright object in a dark sky, so again not a shot I would choose this camera for most of the time.
So the results are unsurprising, at least to me: this camera does typical shots extremely well, but can take 'bad' images if pushed too far or in difficult situations. Its iso noise is better than my 3-year-old K200d (with aps-c sensor) when shooting jpeg, which I find surprising and impressive. If I were holding a 2010-11 Pentax (either k-x, k-r or K-5) the contest would end in a hurry; I had under $200 to buy a camera part though, so those bodies are not avaible, nor is a fast Pentax-mount zoom. I feel that this was a very good imaging investment for where I am now, and for what I usually shoot. It's a keeper.