I went from P5 to P7 myself and found quite a lot of welcome new features. Much improved lighting abilities (though still not 'perfect') very much improved. 

Many onther features also, already mentioned.

As theFixer pointed out, you can add your Poser 5 runtime to the library list so you won't lose or have to rearrange any of your P5 content.

Of course you will need to reset any personal preferences as far as Launching to a Prefered State goes, but that's been pretty much a given when upgrading to any of the newer versions from a previous one.

Hey! His nose is dry! ... Someone should lick it,  just in case. - Diego