Today was my first experience with "alpha" version of Sitecore migration from 5.3 to 6.0.
What I can say, that was not so bad :-). Anyways, I have to say that my site was pretty simple and did not have heavy customization (no custom fields, default devices, no proxies).
I migrated only master database because I had only default security objects in my solution.
I followed all steps from documentation and did not face any problems. Altogether it took about 3 hours to migrate my content and clean up my solution from migration tool. Not bad! Again that was the first time when I saw this tool.
In addition I spent about 2+ hours to adjust my code to spin it in Sitecore 6.0.
The most common issue for my renderings was related to pulling out media url using "sc:fld()" function and media item variable.
I had to substitute "sc:fld()" for "sc:GetMediaUrl()".
I thought I would have to do some modifications to get in-line editing working in V6. I was surprised that old buddy "sc:dot" represented this opportunity.
So, as a result it is not so bad as it sounds at the first glance ;-).
You should try it out!