Had all the K700 K750 & K800 and all suffered problems with the joystick, the W810i is better as that just has a toggle type switch but suffers in the camera dept as its more a music phone (Walkman). Have to say tho they are/were sturdy no nonsense phones.
I currently have the N73 and have to say its a far better phone than the K700/750/K800 and is far better spec'd. Only had it a couple weeks so a long term comparison i cant give yet but as of now it seems to better than the SE's in just about every department both camera wise and player wise and now ive flashed it to the Music Edition firmware the music playing ability is on a par with the Walkman efforts both thro my hifi and car. The SE headphones supplied are far better than the Nokia ones tho. If you buy the adaptor you can use any phones with the N73 which ive done.
My advice would be the N73 as its also 3.2MP in line with the K series and has the Carl Zeiss lense which my camera buff buddies say is quite important.
You may also want to check out the new phones launched just yesterday by SE there maybe some decent camera phones there (im sure there is) although when theyll hit the shops is anyones guess.