I just recently upgraded to an iPhone 5, and it really is a great phone, albeit very similar to all the other iPhone models.
I like how the screen is a bit longer and the resolution a bit better than its 4 and 4S counterparts, but like Mac said above, if you currently have an iPhone 4 or 4S (and want to upgrade to the latest model), I suggest waiting till a newer model is released because you really won't be able to tell the difference between those and the 5th model.
It's highly likely the next model will be much more than an improved iPhone 5. So whilst it's probably worth waiting by now, the changes won't be a whole lot more drastic than the 4S->5, which was a reasonable jump, IMO.
That's the thing with iPhones. They don't have to switch things up every generation, it's an incremental progression. Half of the selling point of iOS products is that they have a well-established, well-known platform. They don't have to keep changing things for the sake of change, which I don't deem a bad thing. As long as it doesn't stall progress dramatically. Bleeding edge is not necessary - this goes especially for quad+ core phones with ridiculously-sized screens. They are getting to the point of heavily diminishing returns, yet the elite outspoken consumers complain if they don't deem it enough of an upgrade.
Look at the Samsung GS4's recent announcement. Here's what most of the articles say:
"I know the screen is bigger and has a higher resolution, the processor is faster, there is more ram, the camera is better, the battery is bigger and the handset itself is slimmer and lighter compared the GS3, but it... it... looks like similar to the GS3 and I'm an over-entitled keyboard warrior who can't actually justify or even quantify what it is that I think
would be enough, so I'll just complain incessantly that they let me down personally."
...and the same thing goes for most major phone releases, but especially the iPhones and Galaxies. As a result, the companies are forced to bump all the numbers up and change the appearance enough to keep the ravenous fanboys and media 'experts' from ridiculing them, rather than focusing on actually improving the phones as a product. Is this why we still pushing the screen size to silly levels and cramming as many cores as possible into a constantly slimming chassis, rather than extending the battery life or improving reception?
I had to get that off my chest - it's not directed at anyone here, I just needed to share my view on the situation.
Oh, I'll answer the question. iPhone 4 16GB Black. Need more space and it can get a little sluggish from time to time, but it's continuing to serve me well. Honestly, moving to a 32GB 4S would keep me happy for a another few years to come, but I'm not up for spending the money on a new phone for the time being.