As Zee knows, I set up the Blackberry service for our work - Ive had both the 7230 and 7100 ('Charm'). The former is a squarish thinnish block just slightly wider than a box of ciggies; this has a full querty kbd. It was primarily designed for email and calendar functions, with phone as a secondary function, and as such was a bit clunky.
The 7100 tries to address the clunkiness by being shaped more like a phone (albeit from the 80's) and swaps a full qwerty kbd for multi-press and predictive text on an 'enhanced' numeric keypad.
In terms of functionality - both are the 'same' and excellent for checking emails/writing quick replies. Document/attachment support is 'basic' though.
All the phone operators have them (except Orange I think), and you can get them from anywhere for 'free' to £279 depending on tariff. There are two options - a corporate solution which requires the IT dept to have the infrastructure set up, or a Web host solution (mostly for home/non-corp use) where you direct personal emails (inc hotmail and other webmail) through it to your BB.
You will have to subscribe to a Blackberry and/or data tariff from £5 a month typically.
Sumamry - if work is giving you one, or you can get one cheap, and dont want a micro phone, get one. Its a great converged device, but does sacrifice the functionality somewhat of individual devices.
Alternative would be something like a PDA with integral phone, like the XDA's or IPAQ 6150 - these are superb mini computers, with phone capability - again slightly too large if you just want a phone.