The Nokia 5800 is available on very cheap contracts, and is also available on PAYG, which makes it an attractive option for a mid range smartphone.
The 5800 runs symbian, which I find a bit clunky, but as it has been around for a while (this is the first 5th edition, but most of the 3rd edition apps I've found work) it has a large following and a lot of themes and apps available.
The latest software comes with Ovi Maps, which is a free satnav app which works very well, but be careful if using it abroad or without a data plan as nothing is stored on the device, which can amount to massive bills.
The battery lasts quite well, and the speaker quality, while not remarkable is good enough most of the time. The antenna is also quite good and does not cut out like the iPhone 4's 'Death Grip'
If you're in the market for a mid-range smartphone you should definitely have a look at this, but I can't help that it would be better if it had Android, but that was never going to happen!