I don't want to go over matters already explored in the iPod Touch articles here and here, but all of these naturally apply to the iPhone 3G.
With the roll out of MobileMe (daft name, great service), any individual can now have a fully synced range of computers and iPhones (and iPod Touches) where all your contacts and calendar entries, Safari bookmarks, along with email accounts are all identical, on all machines. Those working in corporate environments have had this for a while; the necessary thing being an Exchange server (with which the iPhone 3G is compatible). Now, with nothing more than an annual subscription to MobileMe, this service is open to individuals. There are other benefits too like an email address, web space and a virtual 20Gb hard drive (for backing up important documents, off-site).
In use, the iPhone is pure elegance. Everything's intuitive and like most things Apple, just works. After syncing with iTunes, all my email settings were copied over, contacts, calendars and bookmarks also followed suit. The only thing that I had to do manually was choose which music and presentation slideshows I wanted copied over, as the space on the unit is limited to 16Gb.
I'm still getting used to the keyboard, but its not too bad. The way the phone works in perfect harmony with its other apps is very neat. If you see a number on a web page, just hit it and the phone will ring it.
The GPS is quick, and very quick if WiFi is also turned on. It found my position accurately within 15 seconds of starting from cold. The inclusion of Google Maps is very handy and will serve navigating tiny streets in The City well, when away from my car and its Sat Nav.
I also really like the way it stores texts between you and individuals as conversations. No more trying to figure out what one word answers were about.
Safari works perfectly and having a 3G phone now means I'm not in search of hotspots.
With O2, all tariffs include unlimited UK data. Email and surf to your hearts content; you won't be paying a penny extra. You also get use of The Cloud and BT OpenZone hotspots thrown in the deal.
Call quality on the handset is superb. Even with around 3 bars reception, all the calls I made were crystal clear, both for me and the recipient.
Its early days, so I can't comment on how good O2's service is going to be. I just wish that the iPhone 3G was available on Vodafone. Now that Apple's mobile works more like traditional mobile manufacturers business models, I do hope that these exclusive deals will stop and the phone will be rolled out on other networks.
My only other wish at present is that battery technology catches up with the demand the phone makes. Hammering the phone with 3G and WiFi switched on, whilst playing music, games and other apps, making calls and sending texts, the battery got down to a quarter power in half a day. I realise this isn't typical day to day usage, but I would advise always having a car charger handy and never travelling without a charger (or at least the USB charging cable for attaching to a laptop).
Since last writing, I've found a few more apps which are keepers:
AP Mobile News Network - bit US based, but a good source of international news.
Fizz Weather - very accurate.
Wiki Me - Finds articles to do with your location.
Chopper - great little game!
Apple has produced an amazing product. Married to MobileMe, its untouched in the market place. This is even more astonishing when you realise that this is only the company's second phone!
UPDATE: Now that I've had the phone for a few days, I can comment properly on battery use. After the initial couple of days of constantly fiddling with the phone, things have settled down to me using the phone as I would normally. I've kept the 3G and WiFi on constantly and used it day to day to keep up with emails, surf the web, play music now and again and play the occasional game. I'm actually surprised that by the end of the day (around 11pm) I still have around a quarter power left in the battery. As I mentioned, part of this is the device being used as the tool it is, as novelty wears off and also because the battery's had a few charge cycles and begins to bed in.
My second comment is to do with MobileMe syncing. At one point my iCal entries (there were two) weren't being synced. The way I got this to work was switching off syncing on the iPhone itself (Settings/Mail, Contacts and Calendars/My .Mac Account and then slid the Calendars switch to off. Then press the home button). After a minute, I switched it back on and in a few seconds it synced everything up fully. This same trick should work with Mail, Contacts and Bookmarks.