I bought the K-x a couple of weeks ago and have no regrets. I don't think you could do better from an image quality to price perspective. Like you, I bought on a hobby-basis but had an incentive to buy Pentax in that I had two good old lenses from a family member which I could use on a Pentax (and which saved me several hundred £'s if I'd had to buy them new).
I had considered the K-r and actually bought one and returned it for a replacement because of the front focus issue, but ultimately returned the replacement as well. The K-x has no such issue.
From an image quality perspective, the K-x matches the K-r, so the principal differences are: (1) slightly smaller, lower resolution screen; (2) no focusing confirmation points; (3) AA batteries only; (4) 4.7 frames per second v 6 fps; (5) no AF fine adjustment.
(1) I got used to the screen pretty quickly and the screen is absolutely fine for in-camera checking.
(2) Focus confirmation was a lot less important to me in practice than I thought it might be (I centre-focus and recompose). I've actually really grown to like the clean viewfinder and I trust the AF (unlike on the K-r).
(3) The batteries seem to last forever and I got some rechargeables as part of the package, so not an issue.
(4) Not an issue for me - I rarely use this and I'm not sure I'd notice a 1.3 fps difference in practice.
(5) So far, none of my three lenses have need AF adjustment.
There are other technical differences, but none which matter to me on a day-to-day basis. The only other difference which I noticed at first but which I'm now used to is the ergonomics. The K-r fit my hands slightly better, but the K-x feels fine now too.
Regarding other brands, I can't say too much as I haven't used any others, but pass on a few things which I found in my research:
- the nearest current DSLR price-wise is the Nikon D3100. I think it is generally accepted that the K-x has far better image quality at higher ISO (that's my impression from the
Imaging Resource tool anyway). If you're planning on buying any lenses, you may find the D3100 (and D5000) limiting in that it doesn't autofocus all of Nikon's lenses and also doesn't have in-camera stablisation (which means lenses without stabilisation built in will have a greater tendency to blur).
- Canon's current entry models seem to be more expensive, but perhaps a slightly older model might compete on price. I didn't research Canon very much, so I can't really comment.
This is my experience. Hope it's helpful.