Pinky - have a play with mine (older 3GS) or gayboys (4) at the Lakes.
Like any smartphone, battery life isn't great - you won't get the 7+ days you get from 'normal' phones, without really nobbling the functionality.
It seems iPhone is really badly affected in poor signal areas - with my Vodafone SIM, I have to charge daily. With the O2 SIM, mine will last 2 or 3 days, depending on useage. Thats with everything enabled - wifi, bluetooth, 3g etc. Turning these off does extend that quite dramtically, but I can't be bothered.
The iPhone is pretty simple to use, after all, its designed by Apple, a company voted the most gay friendly brand a couple of years back

It has its faults. Signal strength. And I'm not talking about the well publisised antenna issue on the iPhone 4. All iPhones (actually, most smartphones) suffer from 'the grip of death', it was just the iPhone 4 was extreme, and the media got hold of it. That aside, like many smartphones, but worse than a lot of them, it struggles with weak signal. Ironically, I reckon the iPhone 4 is actually better than the 3GS for holding on to a weak signal.
As to buying a second hand one, 3GS is the minimum you want, ideally 4 (iPhone 5 out this Summer/Autumn, so expect there to be a lot of 2nd hand 3Gs and 4s available soon). I would avoid the original (non 3G) iPhone, and the iPhone 3G, these are too slow now.
The cameras aren't brilliant. Usable, but not great. Whilst megapixels isn't everything, there are better camera phones out there. But, no phone beats even a cheap compact digital. For my needs, gayPhone camera is adequate in daylight.
My 2 biggest gripes with the iPhone - and we've had ours for over 18 months now (probably the longest I've owned any smartphone, as they normally peeve me right off) - are its reliance on iTunes (heap of shite - I cannot stress how bad it is), and no removable memory cards. If you have gayPods, then you know what I mean about gayTunes.
Generally, as I drive about the country, I find O2 has the most consistent signal, though not the most consistent 3G. For me, I would only use O2 network with mine - that includes Tesco Mobile and GiffGaff. For the latter, which is what I use (£10 a month, 250m, unlimited txt, unlimited data), feel free to use my sign-up code, where you will get a fiver's credit when you do your first topup (its a PAYG service, where you buy packs ('goodybags') for a month for call/text/data bundles
My referal URL is...
http://giffgaff.com/orders/affiliate/theboyor PM me, and I'll get a SIM dispatched to your address.
Again, anyone interested in GiffGaff (as its not run like a traditional provider) or iPhone, I'll be happy to explain at the Lakes.