Honestly, after playing through most of a 2.02 beta game...I'm liking how this all works. My space Dwarves have been strong all the way through. The 'deliberatly claim' system mechanic is somewhat micro-intensive, but since I often ran out of influence to go grab systems it was micro every once in a while and it really made me plan out what I was going to get next.
I also fill in all of my space and leave no gaps. But that's partially my preference to have strong borders and no pirates and partly an RP thing with the space dwarves being very defensive minded.
Do note that several of the Traditions reduce the penalties for having more planets and reduce the cost/maintenance of stations so growing big is quite feasible with the right planning.
I'm not sure if the AI is getting any discounts on system maintenance, but they seem to have grown fairly reasonably for most of the game. They did push out a bit faster than me early on, but I did focus on a bit of early economic growth rather than rapid expansion, so not sure how much cheating was going on there. As it is, I was about on par for most of the nations near me until late game when my Federation started smacking everyone around us and absorbing them. Except for me, since I've been surrounded by federation members for a long time. So I've gone tall and built a dozen Habitats or so. Currently working on a ringworld as a background project right now. Dwarves do Megaprojects, so I went all in on those.
I *really* like the claim system for warfare. It feels more worthwhile to do small fights to gain bits of territory, and nations gaining/losing territory works much more organically.
Right now we've got the space invader bugs crisis munching on the other end of the galaxy from me, and I'm planning on how to defeat them if they get this far. They might not, there is an Awakened Empire right near them now who might solve the problem for us. If not, I've got Jump Drives and Gateways and the strongest non-AE fleet in the game. And a Doomsday device (neutron painter) that should make quick work of infected planets.
Most fun I've had in Stellaris in a long time!