I like it, and really love that they added web interface (review, overall view of changes, etc.).
P4 is very easy to get started, all you need is to learn several things, and tools underneath can easily be changed, learned to do this for you. Then it's easy to comprehend changes, since all CL's are monotonically increasing, so once gets perception when a CL is announced whether it affects him, or her, etc.
And the elephant in the room is the really, really huge files: psd, tiff, max, maya (ma/mb) models, fbx, zip, iso, exe installs, etc.
There are some peculiarities, like text files handling and conversion (e.g. \r\n <-> \n).
Then some useful (but controversial) features: locking (per extension), or manually per file.
Having all your projects from different teams in one place, but each team/project can customize a view to its own work (a bit opposite of what google is doing with their mono-repo, but unlike g4 there ain't option to sync only what you want, and have the rest off the network).