my friend wants to join the BB group and he wants to know if it is better to get the 8700g locked or unlocked. Will he have problems loading things on it or using the internet/messaging if he gets it unlocked?
What are the pros and cons of getting locked/unlocked?
Locked/Unlocked only refers to whether the device can be used on other networks besides the one it is branded to. If you buy an 8700g locked for TMobile it will be locked to TMobile and you can only use it on TMobile's network. If you buy an 8700g for TMobile that's been unlocked you would be able to put a sim card from Cingular, or another GSM carrier and use the phone with that sim. It means nothing else.
In my experience, an unlocked BB is only good if you are planning to frequently use it with another carrier for voice only calls (and/or have the apetite to go through the entire switching process for data). Even internationally, I have found it to be easier to just let my BB roam, so I can get my emails and voicemail and carry another phone with a prepaid (local) SIM for outgoing voice calls.
The only other way unlocking is helpful is if you purchase a phone that was released for another carrier or plan on making a quick buck selling yours on ebay or something!
It's also useful if you're switching to another carrier that offers BB service (e.g. TMO to Cingular). My Blackberries are Cingular branded but I've used them with both TMO and Cingular and have been able to do so because they're unlocked. Since I'm a BES user, switching carriers is only a matter of swapping SIM's (as long as both have BB data provisioned). It's not quite as easy if you're BIS only since the BIS accounts can't be transferred across carriers.