Synchronizing SVN repository to Mercurial one is deceptively easy. Convert extension is as simple as it gets:
hg convert svn://1.2.3.4/Repo Repo
Unfortunately, this method has issues with speed, especially when there is huge SVN repository on other side of slow (and unreliable) VPN connection. I had seen few hours download times per commit if everything goes well. If something fails, you are back at square zero and you need to re-sync complete commit again. And even if commits are not big, scanning time will drive you crazy.
I found that, under those circumstances, two step approach works best. First we synchronize remote SVN repository to local (SVN) one. In all instances I tried this, SVN offered superb speed (compared to Mercurial) and it hasn't suffered from broken connections as much.
My client of choice was Visual SVN and few commands were all it took to create local copy:
> svnadmin create C:\Repo > echo exit 0 > c:\Repo\hooks\pre-revprop-change.bat > svnsync initialize file:///C:/Repo svn://1.2.3.4/Repo Copied properties for revision 0. > svnsync sync file:///C:/Repo svn://1.2.3.4/Repo Committed revision 1. Copied properties for revision 1. Transmitting file data ...
After local copy is created, you can use hg convert without further trouble.