Quoted from barakandl:i am only working on new boards anymore and water soluble flux is a fantastic.
Water Soluble is awesome, except if it's not cleaned fully it's highly corrosive. Technically not meant for hand-soldering / cleaning.. more for smd at board houses that have machine washers. I'd have switched over to it a long time ago for all new boards, except I tend to err on the side of caution and when washing under water, I go over the same areas of the board multiple times. For smaller boards, time spent there ends up becoming somewhat of a toss-up compared to cleaning off other fluxes. And with the other fluxes there's no worry that what you're seeing will look any different a few months down the road. I keep trying different methods though because cleaning flux is a nuisance. Seems to be no perfect method, everything has its pros and cons.
Here's some pictures of a board that wasn't washed and sat for a few months..
http://www.cheapscience.com/2010/11
Not sure how I'd feel about using it on large boards with a lot of components. I guess though if you're setting aside some boards from various lots you do and sticking to a fairly consistent cleaning method, then after 3-6 months you should have a good idea if cleaning methods are adequate.