I have the 520 which is a replacement for a Lumia 800 that got fubared.
Its bloody good for the price only downsides compared to the 800 it replaced are blacks are not as black on the screen(but you get used to it) and lack of a flash for the camera.
Not bothered about a front facing camera ,never used it on a phone when one has been available.
The 525 appears to only have a increase in Ram(1gb up from 512mb)but from the article I read it will not be available in the UK.
The 625 is better specced with 4G and a flash but the screen resolution is the same as the 520 so is not quite as sharp on the larger screen.
Depends what you want out of the phone, I got the 520 as a PAYG upgrade from Carphone warehouse on O2, but as they come unlocked I’m currently using it with my contract 3 sim(1month rolling contract).
The overall cost works out less and I get more minutes and can opt out with a months notice.
One good deal I saw is the Lumia 920 on contract with Tesco(O2) for £17.50 a month for 24months then £7.50 thereafter.
That’s a bloody good phone at a good price I was quite tempted but don’t want to be locked into a contract at the moment.