Actually OBD2 doesn't exist in Europe, it is a USA spec.
In Europe it is called EOBD and is mandatory on petrol passenger cars from 2001, diesel passenger cars from 2004 and diesel vans from 2007 on. Any car older it will not work most of the time.
EOBD can be a useful tool, but you can only do engines and automatic gearboxes. Anything else like Abs, Airbag etc. doesn't work. Also, you cannot reset a servioce interval or test components. If you want more than just the emission related codes on engines you would have to buy a professional after market scanner.
If you do a lot of VW/Audi then VCDS is a must (dealer level). www.ross-tech.com
If you do a lot of Opel/Vauxhall then Op-Com is a must (also dealer level): http://www.vezerdiagnosztika.hu/index_en.html
For BMW's I recommend: http://www.bavariantechnic.com/