If your going to get one then go for the mosmatic range there're about the best make out,but costly
http://www.mosmatic.co.uk/subcat.php?catid=1&cname=Surface%20Cleanerslike someone mentioned make sure whatever model you get that your machine meets its spec.
I've a couple of pro's and con's about the one i had
Pro's
you can cover a vast area alot quicker
no stripping
no splash back what so ever
less fatigue
Con's (and the reasons why i got rid of it)
you cant go right up to the edge you still end up going back over the area with your lance/nozzle which imo on a very large area can add up more time on a job.
Flat surfaces,means just that,they have a habit of ponding on slighty uneven surfaces,where's the water will pond on a dip and reduce the affect on the nozzles you end up going over the same spot again.
They dont blast the dirt away,if your not on a slight slope you still have to go back and rinse the dirt off i had to end up getting a water broom cos of this
http://www.northerntooluk.com/products.asp?partno=222552E&cm_mmc=Shopping%20Site-_-Google%20Product%20Search-_-Pressure%20Washer%20Chemicals-_-Prod-222552EThey scratch easy even if they are stainless steel
I had the 24'' one was a pain in the a**e havin to keep moving about in the van they take up a bit of space.
All in all,by all means get one,but in the end i bought a twin turbo nozzle lance and found large areas just as quick using this instead
It's up to you,these are just my opinons