Within the last year I have become addicted to subversion. I am an only developer and I also work on a few of my own projects. With SVN its really easy to manage everything
Rule no. 1: "Never change a running system"
Also, as there are many shiny new solutions(for problems you do not have, as you work alone) you should consider the cost of switching to a new VCS: The import of subversion to Mercurial/git is not an easy task
there is no tool (AFAIK), which imports svn repos by using the dumpformat. So if you do not use the dumpformat you will stick to checkout all branches/tags from svn and adding them to git/BZR/Mercurial manually/via script
So I do not know how big your repos are(my repos are ranging from 20 MB to 24GB), but it will take a long time to check out a whole repo and even small projects with lot of tags will eat up a lot of harddisk space.
Additional problem is the time until your migration is done you cannot continue to work.
There's an old Yankee proverb.
If it ain't broke, don't fix it.
I also personally would stay with Subversion, Is there anything better?
Subversion is a great version control system, and you're happy with it, so if you are looking further, I can recommend you to get some info about Continuous Integration, there are many tools out there that can help you to make automatic builds, make your builds self-testing, check the integrity of each commit, and much more...
If SVN addresses all of your needs then I don't see the reason for change. If curiosity is the driver of your quest for a different source control then I would recommend reading about git or other distributed scm solution and try to figure out if it's worth the investment in order to switch (which i doubt it is in your situation).
I am just like you in the issue of constant investigating in order to get the best tool.
I tried SVN for SOLO work and someone recommended me Mercurial (hg). Now i do keynotes about it. It's more friendly than git in windows. I now i think "why do svn complicates with simple task like tags". SVN doesn't know what a tag is. For SVN a tag is a copy. In mercurial a tag is an alias for a revision. How complicated could it be?
Performance it's an other issue. In Mercurial your repo it's in your local machine. So it's very fast for a log, or diff or history.
Although I do not know anything about servers that support mercurial for a online version of your repo.
It's definitely worth looking into "distributed" VC even if you're not actually using a distributed workflow. Being able to have private branches and control over your local commits is worth the effort of learning git. I've been mainly using git-svn (with other team members using regular SVN clients, so we had a normal, centralized workflow), and it worked pretty flawless.