I've had one for about a month now.
It has its good and bad points. Basically it feels like it has been rushed to market too soon and has glitches, software issues.
The Good:
Looks fantastic, even better quality screen. Everyone comments how great the image quality is compared to othre phones.
Nice and simple menu system, refreshing after ANdroid in many ways, you have to use it to understand why.
Very fast, no glitching, freezing, slowness in operation.
All the apps I used on ANdroid are available on Windows, even though people imediately say dont get a Windows phone because there are no apps!?!?
Automatically uses wifi over 3G when available.
The battery last longer than my HTC Desire did previously, thats even leaving things on like GPS (however there is still a huge battery problem with some handsets ie only last a matter of a few hours)
Cons
Mine keeps crashing when I come home when the phone switches to wifi (not every time but most times) going to contact the network and see what they say..
When I first bought it I had a huge list of cons, but to be honest as I get used to it the cons list is getting smaller!
Big con. Not all settings can be attached (pinned) to the main menu. I use the sat nav alot, and there is no way of shortcutting the brightness setting to a menu.
Syncing with a PC is a nightmare (well I think it is) You cant just plug a lead in to a PC and do a direct transfer. You have to use software called Zune and its a real pain.
Everything is syncd via Micrsoft using Hotmail and clouds etc. I dont like the idea that all my contacts, pictures etc are held some where else and not on my phone or my PC.
Endomondo - It will not run in the background. If you receive a text or email it interupts Endmondo from working, very frustrating. It could be an Endomondo error, but its frustrating.
No flashing LED for received messages/calls/email. I miss this alot from my Desire. + Ordinarily you can not see how much battery is left, whether your connected to 3G or wifi, and the clock is tiny.
The charger lead connects under a stupid fidley cover on top of the phone, which is a pain and you can see it breaking easily.
No way of monitoring 3G usage!
The camera is OK, and produces nice results (for a mobile phone) but the menu system is not the best, and the zoom function is terrible. ie 3 settings wide normal and close.
Dont get me wrong I am enjoying the phone a lot and prefer it to my Desire by a long way. Unfortunately it just feels unfinished in the software department as though some things have been forgotten about, or they havent had time to finish.
Edit: As to using in stead of a tablet, I'd want a bigger screen, but then you might as well get a tablet. I use mine for web browsing etc infront of the TV just like anyone else, but use the netbook if buying things, need a decent size screen. Social network integration on the phone is second to none. In fact too easy, and can be difficult to stream out all the crap people post! Its only 16GB internal fixed storage, so that wont go too far if you store a load of stuff.