Have been thinking about this for a long while, but whats always stopped me from moving was the partial check out feature. I keep all kinds of files in SVN (word, pdf etc), we are not going to debate whether it is good or bad, but it is convenient and thats all that matters.
The problem with the SVN thats HUGE, is that everything takes ages, and a broken directory somewhere can lock the entire repo (me using a monolithic repo for everything = very very bad now that I look at it). By moving to Mercurial I am going to look at creating multiple repos for every project. This has advantages of its own, for example you check out only what you need (so I still in a sense get partial check out of my repository), and things are likely to be faster and more stable..
Moving to Mercurial is going to be a long process, and I plan to migrate only individual projects when I touch them. There will most likely be a whole series of posts on Mercurial.