Hi
Got mine today too - an 8GB one again on Orange from
http://www.e2save.com (CWH really).
I agree about the optical mouse. I would switch it off but I like it for web browsing. In the menus it can be a bit annoying but it may just be a case of getting used to it and I think you've got to discipline yourself to *only* use the optical joystick for moving around the screen. I've switched my desktop to show the icons in a row across the top (Settings | Personalisation | Standby mode | Standby theme | Full page) like I had on the N95 which isn't as bad with the optical. Ironically, I actually find that an increased sensitivity is a bit better (Settings | Personalisation | Optical Joystick - changes horizontal & vertical to 7) as at least it's more predictable. Ideally, there would be a setting saying only apply it to web browsing.
It is a shame about the GPS. On the N95, there was Nokia Maps and it was pretty good (especially after a firmware update brought in a-GPS) but on the i850 it just seems to have Route 66 BUT you have to pay to get maps of GB (£50 and a 230MB download too) whereas with Nokia Maps already had GB (or at least it downloaded them as you needed them maybe?. I also don't know if Route 66 has the feature where you can see your position on a map of the local area and as you walk around your position changes on the map in front of you - this was a very useful when I recently went to Weymouth and lost my bearings and wasn't sure which way to head back to the hotel - I had the map open as I walked and it steered me back perfectly

.
There is Google Maps which aren't bad but they are not as dynamic as proper GPS mapping software. Also, when I initially started using it, it was causing the phone to reboot but seems OK now.
My biggest bugbear is still the way applications on the S60 platform can end up still open in the background using up battery power. After having the N95 for over a year I still would occassionally find something open I thought I had closed. On the N95 you can press-hold the menu button down to get the running apps list and from there use the [ button to shut one down. On the i850 you do the same thing but there is no [ button - the end call button is supposed to exit the selected application but I find it doesn't always work, especially when you have a web session open. If you press-hold it, it disconnects the net connection but still leaves the browser open? Anyway sussed out a quick way round this, other than having to go back into the browser and use the Exit option in the menu?
Anyway, it is a lovely phone, much sexier than the N95 so am happy

So, after all that waffle, here's an image of the i850 next to my N95 - if you thought the N95 was big enough, think again! If you thought the N95 was heavy enough, then the i850 is 16 grams heavier too. The i850 is a little thinner though:
