I am a web developer (J2EE application developer) and just want to expand what tools I use. I want to use Open Solaris for my personal projects. I have nothing against Linux and It looks like a lot of the same tools are on both systems.
Have you jumped to Solaris, was it a good experience?
I guess the underlying OS doesn't matter much for a J2EE developer, as long as you stick to the java platform and don't make use of native libraries through JNI. Having said that, the most important factors to choose an OS would be cost and performance. Now, both Linux and OpenSolaris are open source and free to use, but I'm not sure about using OpenSolaris in commercial deployments. I also don't know how java performance differs from one to the other, but I'm strongly convinced that Sun's implementation for Linux is damn good.
Note: I've never used OpenSolaris and I use mostly Linux.
For J2EE work per se, probably not much. As a more general developer you may appreciate DTrace. As an admin you'll love ZFS & zones. You'll hate the outdated utilities (mostly user-land) though. FreeBSD is a nice in-between Linux & Solaris though. :)
I'm not certain from your question if you mean for your development desktop or your hosting solution but I can take a crack at both. About six months ago I got hold of a free year of hosting on OpenSolaris running GlassFish. I hadn't used Solaris before and thought it would be a good learning experience. I built a test server, installed OpenSolaris and GlassFish, and used it to practice. It was very strightforward to configure GlassFish and deploy applications. Managing services in OpenSolaris is also simple once you read the right documentation. I like OpenSolaris and I like GlassFish.
Obviouly, I found similarities and differences from previous experience with Java application servers and operating systems. However, I thought so highly of the OS that I switched my desktop over last month. It has been a good experience.
DTrace, zones, switch between 32 bit and 64 bit mode with a single GRUB switch, ZFS, stable libraries (I can't really emphasize that one enough). Solaris 7 software generally runs on OpenSolaris, otherwise known as Solaris 11. glibc changes between minor kernel releases.
Xen is integrated pretty tightly, and setting up lx zones or virtualization to keep your Linux environment is dead simple.
OpenSolaris now has /usr/bin/gnu, where all you favorite utilities can be found.
Expect, though, to end up fighting the ./configure && make && make install cycle a little bit. A lot of developers assume you're running Linux, and don't prepend -m64 for Solaris, among other things. Compiling wxPython is an adventure, for instance.
Edit: I forgot to mention one (possibly important) thing to you. Package repositories aren't nearly comparable. It's neat that pkg image-update (equivalent to `apt-get update && apt-get upgrade && apt-get dist-upgrade) makes a ZFS snapshot that you can get back to via GRUB at any point, but you have nowhere near as many packages in IPS as apt. All the biggies are there, though.
If you're planning to switch, Sun's documentation is fantastic, and the BigAdmin tips of the day are worth reading for a while to get you up to speed.
Eclipse is not available on OpenSolaris, unfortunately. If you are an Eclipse user you would have to migrate to NetBeans.