The 8520 is a great phone and this offer on T mobile should be very good value
I contacted T Mobile before I pre ordering this phone to ask if I could use the 8520 with the Blackberry Messenger on Pay As You Go and their customer support said they dont support this phone on payg

Could Play,com or the supplier of this phone please try to clear up the confusion that surrounds the Blackberry phones on payg or guarantee the T Mobile sim card provided with this phone can be used to subscribe to the blackberry messenger service

I have this phone on the orange network. This phone is fantastic, it has a messenger which allows you send messages and pictures to other blackberry users for free.

Only problem that I had is that when I got this phone the internet would not work. Although as part of my contract with Orange I had unlimited mobile internet, Orange told me that I needed to pay an additional 5 pounds a month as blackberry uses a different type of internet to normal phones. It is connected all the time to allow emails to be recieved where as normal phones have to connect once you request it. This 'add on' included unlimited texts which is something to keep in mind if looking at it for a contract phone.

Definately Worth Buying, its the best phone I've ever had! Great keyboard, cool features and the Pay as you Go works great with the 10 quid a month unlimited texts and web service T-Mobile offer. I can work my apps online, use facebook and send all my texts. Bliss.

THIS IS AN AMAZING PHONE! seriously, i would recomended it to anyone who is looking for a phone perfect for email, the internet, facebook, msn whatever you are looking for, you will find it in this phone. the camera is good (in good light) and the BB world applications are great.
Im so happy that t mobile have introduced this on PAYG, which means that for only 5 pounds a month you can get unlimted internet and the Blackberry email services! (the info comes with a sim inside the box) All i would say is that once you get this phone, you will not be able to imagine your life without it!
The only drawback was play's appaling delivery system! i pre-ordered the item ages before its release, so i was expecting it to arrive on the release day (which has changed 3 times already), but then i have actually received it after a week from its release! and with no apology whatsoever.

To the previous review... Yes Tmobile are realising this on PayG but they can not give a confirmed release date which is why it keeps changing.
Once released Tmoile will support BB IM, Theres no info on there page as it has not been released yet.
I had The same concern and called Tmobile who were very helpful.