It's certainly the end-of-the-line for the Sony ereaders in the US, with the last major retailer for Sony, Best Buy, closing out their stock. It seems that Sony just can't take the heat in the US marketplace anymore.
I was looking at getting a 650 for my mom, who liked my 350 but found it too small, but luckily they have been almost impossible to find since last year. I say luckily because it looks like the new ST Nook will be much nicer than a 650 or certainly the 350 for her needs. I still need to see some actual reviews from MR members before I buy it, but if Nook got rid of the "side-load ghetto" along already strong epub support, IR touch, dictionary and pearl e-ink screen it should be very nice. All this at a decent price.
The ST Nook also seems to have nice font/font scaling support, which is very poor with Sony x50 software. After having my 350 since last year, this is my biggest gripe. Sometimes I can work around it by using Calibre to tweak the epub, but this an unnecessary pain-in-the-butt. As soon as PRS+ is really stable, I'll give that a try as it fixes this issue, even though it's somewhat scary to flash a 350.