Cheers mate. In what way is it not the most user friendly or reliable? You mean physically or the user interface?
The interface is annoying and not very intuitive for all sorts of things like saving the number of someone who rings / texts you, choosing the person who you are sending a text to, using the WAP browser etc.
It is also (virtually) impossible to get the phone to ring and vibrate at the same time - it's one or the other or vibrate only then ring only.
The main bug I have encountered is that sometimes if someone calls it becomes impossible to answer the phone, reject the call or switch the phone off and it rings and rings for about 2 minutes. This is very annoying in public because you look like an idiot when you keep saying "hello? hello?" and the phone keeps ringing. Admittedly this only happens in really hot, cold or damp conditions.
No problems with the phone physically regarding build quality. Only annoying part is that there is a tiny plastic part that you have to remove to charge the phone and then replace afterwards. This is very easy to lose

Is it possible to use the rest of the phone while using the mp3 player (which I think you can with the k750..)?
No. The memory is not huge either and can't be expanded.
Also what's the sliding mechanism like, not too fiddly is it?
No problems there, pretty robust and easy to use. Only time it is annoying was when I was wearing a wooly hat over my ears. I slid the phone up to answer and then accidentally slid it back down when it hit my hat as I put it to my ear. Other than that it's a fairly nice feature but probably a gimmick as it doesn't do anything that green and red buttons don't.