I have been using AWN for a couple years now with Ubuntu and have been really happy with it. It's a pretty solid dock and has all the features you would expect. I thought that are other things out there and that I should try them out.
I started with Docky since well... I already had Gnome-Do installed so it was next. Docky is a very minimal dock it looks nice if not a little plain. I started adding icons which was pretty neat you just use the search feature and there is a little plus symbol to add it to the dock. Yup and that's where the fun ended I couldn't figure out how to add a custom icon to an app. which I'm really picky about. The solutions I found were convoluted and very indirect. I continued with it for a couple days and kept it simple. I liked the window grouping and being able to use the scroll wheel to change window focus. I know a lot of people think but "Gnome-Do uses Mono and Mono is the devil" lately but big deal it's not a problem right now so forget about it already.
I Moved on to Cairo Dock.
All I can say is WTF! What a mess. Don't get me wrong there are some nice features like the app grouping and the icon themes are pretty cool but also very incomplete. Applying the themes seemed a bit buggy at times too. It had about as many features as AWN just a bit disorganized.
In Conclusion I'm back to AWN for now and I'm glad to be back. They all have their strengths and weaknesses. AWN gets my choice for two reasons, features and the simple configuration tool (AWN Mangaer).
Quick Update 7/21/09:
I have been unable to use AWN for a couple weeks now due to some bug in Ubuntu Karmics GTK 2.0.
Details here. So I tried using Docky again to find out it was out of date the new V.82 is much nicer than what I was running and it has some windows 7 looks about it but it's pretty nice. It may be hard to switch back that is if I try to.