Hi slender, Gmail (the main version, with cool graphics and lot of features) works, but do not expect anything special. I mean, you can read your mail and use features, but it has some problems in handling server request. E.g: if you press the Archive button for storing a mail in the Archive, it will appear stored, but next time you'll open Gmail you'll find out it is not. The HTML version works well, and of course the mobile version has no problem at all.
The best way for using online Gmail on your N900 is definitely the HTML version.