<p>web2py to is a Python framework but shares the  "convention over configuration" design that Ruby on Rails has. On the plus side it packages a lot more functionality with its s standard distribution and we claim it is faster and easier to use.</p>

<p>Has any Rails user tried it? What is your impression?</p>

<p>No rants please. Just technical comments.</p>

I would say the biggest "con" of using webpy over Rails is that there are not a lot of Rails-specific hosting services around, and the huge community based around it (there are Rails plugins and tools for.. everything). The same cannot be said for web2py.
It depends what you want to do with it - if it's something to write your personal site with, and you already have a server to host it on, use whatever you prefer. If it's something to distribute for others to run, Rails has more options for hosting, and a bigger community, so it may be a better choice.
Technical differences are rather irrelevant. Every framework can basically do the same (generate web-pages). What is important is community, ease of use, useful feature-sets, ability to host it and so on - and those are all really subjective.
I still use PHP quite often, not because "it's better", but because I can host it on a huge majority of web-hosts. I also use Rails because as it has a good, and very active community. The actually technicalities of the framework wasn't ever a consideration, really..
I could probably put together a list of why web2py is "better"/"worse" than Rails - Rails may be 0.04sec/request slower at generating templates containing loops, or web2py may have a good DB model generator, or some other technical reason -  but those may not be relevant to you at all