http://fastrunningblog.com/forum/index.php/topic,587.0.htmlPrevious post on it.
I did it for 3 weeks right before a marathon once and found it kept me in great shape. If you work hard, you can equate it to normal running effort (i.e. 8 min mile, or whatever). And I highly suggest intervals rather than steady- see:
http://coachdeanhebert.wordpress.com/2007/09/25/is-aqua-jogging-any-good-for-real-runners/ I also did ladders (1 min hard, 30 sec rest, 2 min hard, rest, 3 min hard, etc)- breaks up the monotony.
The only trouble I had was that, although I had my fitness, my legs were a bit soft from not having any pounding, which slowed me a bit in a marathon at the end of the aqua jogging.