Firstly this is a great machine, but it's pointless buying the 500gb version, and I'll tell you why. Also I have a cautionary tale.
Why Buy The 500Gb Version? This is simple. The specs are identical for 320gb vs 500gb machines except for the hard drive and processor. The speed differences between them are so small in my mind the additional money is not worth it, but the bigger hard drive is. It is so easy to install hard drives in these unibody macs that you are much better off buying an internal 2.5" 500Gb drive from Amazon Toshiba MK5055GSX 500GB 2.5-Inch Internal SATA HDD - I suggest this simply because I know it works. If you are worried about if you have the skills to do this read on - it's easy! (see vs non uni-body pros section)
Cautionary Tale: I was the proud owner (technically still am) of a dual core 2.4ghz macbook pro, but after just 3 years of daily use the motherboard was fried. Sadly this is not uncommon, and a friend of mine, also owning a similar spec pro, recently had the same problem. The problem with them, and with this mac as well, is that they are not great at re-distributing the heat they generate. There are no vents to push hot air out, and if you use your mac in clam-shell mode this is compounded, as the only open space on a unibody mac is the speaker grills. It is worth bearing this in mind when buying one, and I would suggest if you can find applecare at a competitive price (shop around as there are sellers offering deals) it is worth it. A new mother board is a lot of money. So I no longer take the Pro to watch iPlayer in bed, as the Duvet acts as a warmer which clearly something this machine does not need. I also religiously use a laptop stand with cooler fans - just not having the whole back of the mac in contact with a wooden desk seems to help too.
Why it's Awesome: Clearly these machines must be awesome as despite this I've just bought another. Even though my last Pro had a much shorter life than I expected, it was a much more pleasant experience than my previous foul-mouthed PC days. Unlike PC's which fill me with rage every time I get near them, the mac is simply bliss. Although I'm always left wondering if I could have made-do with macbook rather than the macbook pro - but I use a lot of photo editing software simultaneously with things like dreamweaver and many others, so probably need the power. The software is great, and the performance fantastic, and they are gorgeous - what more could you ask for?
VS non Unibody Pros: The unibody design is a thing of beauty and practicality. It feels so much more robust than my old machine and also significantly thinner. It also means that it is difficult for dust to get inside the machine. So whilst it causes some heat issues, it reduces the likely hood of dust causing component failure. It also means that when you are taking it out and about with you, it is better equipped at dealing with the harsher treatment a laptop inevitably gets. The new track-pad is great, but I must admit I prefer the old layout with the trackpad and the buttons - although the gesture control is handy (no pun intended).
The best thing, in my mind, about the unibody design is how simply they have made it to add and to remove a new hard drive (as mentioned earlier). If you tighten and loosen a screw then basically you can install a hard drive. They even give you the instructions on how to do it the owners manual. I already had a 500gb drive from a previous machine so I simply took it out and popped it in the new Pro. Thus I now also have a handy 320GB spare drive. All you do is take the back off by loosening a handful of screws and then you are looking at the hard drive. Take off the black restraining bar on the side by unscrewing two screws and lift it off. Simply disconnect the SATA lead from the top of it and lift it out. Take off the four screws on the sides of the hard drive you removed, add them to your 500Gb drive and follow the steps in reverse. Simple.
The only down side, and reason for 4 stars is that you can no longer replace the battery yourself. If it goes you will have to take it into an authorised centre and get a new one put in. On the plus side I get about twice as much time on new mac vs the old one, although no where near the 9hours.