The technique is cool, but in this case the customization doesn't seem very helpful. The default buttons are more comfortable to use, I think, although a flat color background or a rounded button or something might be nice. A gradient on something so small, on the other hand, is overkill. (I don't mind all gradients; a soft gradient in the background can have a good effect.)
I do like the way they regrouped the buttons on the toolbar, but I wish there was a little more space between them. Put a few pixels between the buttons in each group; then, perhaps, put a slightly different color behind the buttons to emphasize their grouping. It'd also be nice if the "more actions" dropdown was right-aligned.
I really like the interface on the "Move To" and "Labels" buttons. The "move to" option doesn't seem to fit in very well with the whole label-based interface, though. It's much more intuitive to label something and then archive it (it's just one more click/keystroke). "Move to" doesn't add any new functionality; it does makes label+archiving slightly more convenient, but it clutters up the menu, so I'd say remove it.
And once the "move to" button is gone, it probably makes sense to move the input box that you get when you click "label" directly onto the bar with the archive/report spam/delete buttons. Add a dropdown arrow to get to the list of labels/management stuff, and put "Labels" grayed-out in the input box by default to clarify what it's for.
The easiest improvement, I think, would be to iconify the Archive, Report Spam, and Delete buttons. That would be fairly easy to do, and it would make it easier to distinguish the function of each item and declutter the menu.
The last thing I'd like to mention may just be a matter of taste, but I definitely preferred the older color scheme. There isn't a big difference:
MAIN BOX light blue light blue
RSS BAR lighter blue darker blue
READ ITEMS lighter gray-blue light gray
This change in color scheme totally switches the "look and feel" of the app for me from a pleasant, Google-y environment to a corporate, enterprise-y one. Maybe this was intentional, but I'd like to change it. There is a "classic" theme available, but it doesn't quite get the colors right and regardless I use Google Apps and Themes haven't been rolled out to me yet. For now, I'm reverting to the "older version".
Overall, I love Gmail. Just a little constructive criticism ;)