The answer is, inevitably, maybe. It depends how much the thought of an 18 month tie in frightens you.
Looking at the relative cost breakdowns, and how much the PAYG tariff will end up costing you in theory, per month, it comes down to text messages. Browsing is included in both tariffs, and minutes are roughly similar (more on this later, but basically, it's 1000 minutes on PAYG vs 1200 on contract).
But, there's no free text messaging on the PAYG contract, where as you bag yourself 500 on any contract worth more than £35 a month. On PAYG, you pay 10p per message, so that same amount will cost you a pricey £50. Hikes the cost up considerably, no?
There's an increasing number of ways to get round it if your friends are tech savvy - you can email, twitter, smoke signal etc.
But essentially, if you go for a £399 PAYG and top up more than £30 a month, you're looking at an equivalent £63 a month, with unlimited calls and no texts. Pay for a £45 contract, plus the £5 per month equivalent for a 16GB iPhone, and £50 suddenly gets you 1200 minutes and 500 texts. And visual voicemail and call merging.
BUT!
There's one more BUT!
PAYG free calls? O2 to O2 and landlines only. Contract? Anyone, as long as they're in the UK (excluding 0845 etc).
So, what's it to be? Are you tempted by the PAYG model? Does this give you the added push to jump on the iPhone bandwagon?

I'm never quite sure why Brits love PAYG so much, even for smartphones -- some people have even refused to buy an iPhone from O2 until it's available prepaid!
To me, a device that advanced needs to have all its features open: browsing, e-mail, text should be included and hopefully unlimited when possible. You can use an iPhone on Wi-Fi, of course... but that reduces it to an iPod touch with a microphone, doesn't it?
Still, this is likely going to help adoption, and it's a better deal for those people who rarely take calls but would rather not have two devices in their pockets.
Browsing the internet is free on the PAYG deal, which is a massive leap forward for PAYG customers who've been used to substandard offerings.
rip off!! rip off! rip off! Dont get me wrong, I'm no Apple hater - I think the iPhone is great! But come on, £350?! Surely this won't make it appealing to the mass market? Even at Xmas!
£350 is a lot. If I was going to buy it, i'd wait til this iphone can be unlocked and use my Three sim in it. o2 are a total rip off.
But I think i'll wait for Android.