It has been almost a year since Wolfram's Mathematica 6 was released. All this time I was quite happy with my licensed Mathematica 5.
However, I got tired of sending password requests to Wolfram. You need this every time when you upgrade hardware or reinstall operating system or change your hostname. Every time you need to print some form, fill it, scan and send back (this is the most simple way, if you're outside US). While you just can enter your MathID to a tiny program and it will pleasantly generate password for you.
So, if I lose my license anyway, why not to try new version?
I'm not a heavy Mathematica user, so I'll describe just first impressions. The first thing that Linux user notices — new graphical user interface. Now it's based on Qt toolkit rather than motif, and it certainly makes UI much more usable. Although this is great improvement, it's also the last one I have noticed.
Now problems. Mathematica 6 uses some unreadable font by default. Setting other font in Preferences does not seem to help. But I still believe this problem is solvable.
Now the problem which isn't. Earlier versions had excellent documentation system (help browser). It was very intuitive, easy in use and fast. Just look at screenshot. You choose category (like "Numerical Computation") and see what subcategories or topics it has.
Mathematica 6 made a huge step back. They introduced hyperlink-based index. So, you click on a link for some category and see a page with links to subcategories. It is inconvenient, and it is damn slow!
I used Mathematica 6 for a few days and it's still unclear to me what will I choose — clean user interface or nice documentation system.