Contrary to it's description as lightweight, I have encountered significant hangs on 2 machines, running 10.4.11, and 10.5.7.
Transmission is visually pleasing, and very simple, so i still use it for quick grabs - however it's still quite limited in comparison to Azureus. Most of these features relate to organizing lists of 25+ torrents, or selecting files to DL inside torrents with 25/50/100 files.
Features I miss:
-Click sortable columns when picking files for DL
-Keyboard navigation when picking files for DL
-Network activity graphs with history
-Manually kicking/banning peers
-Viewing selected groups of torrents
-Very compact torrent listings

+1
Very similar to utorrent but has more features.

Download speed does depend 99.9% on the seeders/peers connection....that's why a good test uses THE SAME torrent with a LARGE amount of seeds so that that's not a limiting factor. This way differences on speed can't be blamed on the torrent as they're both the same (basic scientific method 101).
Anyway, good test and I can confirm that Transmission starts up a bit more slowly but otherwise has more features.

miknos, if you want to browse the web and download torrents at the same time, you should try Turtlematic: http://osx.iusethis.com/app/turtlematic

Download speed depends 99.9% of the seeders connection.

The difference in torrent apps today is the features (and UI). Transmission is the best without a doubt.

What really is pissing me off right now is (in version 1.76) the LIMIT UPLOAD SPEED not working! When you open a website, you send a request, and if 100% of your upload bandwidth is being used, the website will take LONG to open! Now I can't browse while having Transmission open! Let's wait for the next update.

I thought I'd revisit this app after upgrading to Snow Leopard and finding that Azureus 2.5.0.4 won't open files/run correctly. I also tried µTorrent ( µ ) so this is a comparison with that.
Firstly, I think that Transmission ( T ) has a better interface and µ appears to have ripped it off!!! (even down to the icons in the prefs menubar). More importantly, T has many more features and also runs in 64bit mode (which µ can't do, only 32bit for that).
The main features different to µ include the ability in T to use the bluetack blacklist to ban bad traffic - essential for any good torrent app imho. Other nice features include the ability to selectively download individual files easily (I think you can do this in µ but only by going into the torrent's info after it's started up). The autosize window feature in T is also great imo (it's the little things that stand out)…

…What about resource usage? About the same CPU for both but more ram usage for T for me (39Mb vs 11Mb) but than again, as mentioned it runs natively in 64bit. Besides, on a new mac, resource usage for both is still negligible compared to, say, Azureus/Vuze.
Finally (& most importantly) how about speed? I chose a 100Mb Audio torrent with >5000 seeds and about 500 peers and downloaded with both. I made sure both apps had identical settings (even down to specifying the same port mapping) and ran the torrent twice, one after the other.
I found that µ took 2min start to finish and T took 2.5min. After looking closely at that, I found the extra 30sec was on startup with T (whereas µ started the download almost immediately). If timed from the moment download traffic appears, then they are the same. I think the slower start may be T loading up the bluetack filters? …but I think these are essential so don't mind the extra first 30sec (and on a bigger file it'll be negligible)…

…In summary, T has more features, a better UI, can utilise blacklists and runs natively as 64bit in 10.6; A uses less memory and is slightly faster to start up.
I've since switched back to Transmission (bye bye Azureus ).

1.75 is highly unstable on Snow Leopard. It crashes often

Indeed, with the addition of DHT support in version 1.7, there's no longer any need to use uTorrent or Vuze etc. to download those difficult torrents. The minimal interface here is simply OS X heaven, yet the preferences and group support allow savvy users to get exactly the behaviour they want 99% of the time.