good chemistry is complicated,
and a little bit messy -LW
You'll miss out on learning a lot of perl. Yes, you can learn this other ways. But having to actually learn how to build an OO class (perl module)- is priceless experience. Of course, it can be argued you shouldn't have to learn what you won't be learning right away
Moose does not hide anything but the arcane OO mechanisms, so you will not really miss out on learning all that much. Perhaps you are referring though to horrors such as manual type-glob fiddling, AUTOLOAD and some of the crazier OO techniques described in Damian's book. In which case I think your point is debatable and many people consider the usage of those techniques to be ill advised for even the most skilled of perl programmers.
Some people complain that it's slow.
To clarify, the startup time is slow, runtime is not. Moose has to bootstrap the entire object system and create several meta-level objects for your classes. Once you enter the runtime though, Moose is very often comparable to the equivalent hand coded Perl and/or slimmer modules like Class::Accessor and Class::Accessor::Fast.
Some people complain about the million dependencies it has- and installing them.
Actually it has 4 non-core dependencies, I explained in some more detail here. As long as you have a reasonably up to date perl, it is really not that bad. The worst is when you need to update Test::Simple and Test::Exception, as they have long dependency chains that can get ugly. But this is really not Moose so much as the Moose test suite.
It's in development
True, but I would think that would be a good thing since you want active development in your tools and not bitrot. Moose is highly stable and has been so for a couple years now. It is used in many large codebases in many large companies. We are very responsive about bugs, and have a pretty large test suite. I also consider the 550+ other CPAN modules that depend on Moose to be our "external" test suite, so if you factor those in, we have 10s of thousands of tests. Also factor in the recently ported Catalyst and all the apps that run on it too.
In short, I would not worry about Moose's maturity and stability at all.