SANOOK  To be fair to the app, it is an X11 app. Native would be nice of course, but for the people this is directed at functionality trumps beauty.  
(Version 1.2.2)

SANOOK  Very nice display of the current month. Developer is very responsive - I sent an email asking about an option to change the font; got a quick reply and it is now an option (ver .63). If you want a calendar view that is a bit out of the ordinary (along a line rather than week by week), this is a good way to go. Source code is also available to those able and willing to tweek even more. Only thing I'd like to see would be vertical orientation (I somehow remember that it is possible, but can't figure out how to do it).  
(Version 0.63)

SANOOK  2.8.0.438 (beta 2) is terrible...crashes all the time. Beta 1 was more stable, not sure what happened. I'd stay away from this and use 2.7 for the time being.  
(Version 2.8.0.438)

SANOOK  This is a good app, works well as advertised. It has nice features, such as the ability to have more than one account polled, nice interface with the ability to change font sizes, do some special views with gmail (like changing the gtalk location, label location, some others), and is relatively quick, too. It seems to be updated fairly regularly, with a fair trial period (30 days free). Advantages over fluid (or even just your browser) are the multiple accounts "active" at one time - most don't let you do that, and (I believe) google even logs you out automatically if you are logged in to more than one account at a time on a browser. So props to the author for getting around that limitation. I don't think it is really correct to compare to Mail.app, because gmail allows you to do a lot of things (labels, access to google docs, etc) that you can't do in Mail.app. If you just want to download your mail to Mail.app, then by all means do that. If you like to use the native gmail interface this is a good way of doing it.

...BUT...

$24-$25 is too much for a one trick pony, even if it is a beautiful PINK one trick pony. gmail browser is decent, but again, only one account at a time and you can't do any special formatting tricks, but is free. I guess I'm just a cheap Mac user or something, even though I really do like this app. Still, I think it deserves a good raiting, because it is a very nice app.  
(Version 2.0b8)

SANOOK  To the two previous posters: read the requirements section BEFORE YOU TRY AN APP OUT. 10.5 is required, it is required for a lot of apps now, so deal with it.

For those with 10.5, the app works as advertised, albeit with a very limited set of conversions. Since it is in beta, I imagine more will get added eventually. The app is OS, so I think you can download the source and add in your own conversions, too (not that I've checked). If this app had Stress/Pressure conversions, I might use it more often.  
(Version 1.32b4)