i3 owner here. The SDD space makes for a bit of "thinking". It hasnt really been a huge problem. With a fast enough MicroSD card you run quite a bit of software just fine if you want to avoid installing to the SDD. I typically just keep most stuff saved in the cloud to also save space (and obviously so that they're available on all devices).
Horsepower has definitely been enough and even much more. I constantly keep wondering how sometimes the SP3 outpaces my gaming PC sporting 3,5ghz hexcore. The SSD does a lot, though. But I don't do any heavy software like.. CAD or Photoshops and whatnot, so I guess it largely depends for that. What comes to the PC usage of vast majority of people, the SP3 i3 has enough power and ALOT to spare.
I do wish it would've had that 128gb of SDD space so I wouldn't have to worry that much about not installing much on the SDD, but you kinda have to ask yourself if you can go around the "challenge". If for whatever reason you need a lot of software installed and a lot of local files, maybe you need more space at least, but once again a higher class MicroSD card often comes cheaper than bumping up to next SP3 model.
EDIT: Skimmed over some other posts here and figured I mentioned that for me the SP3 is my PC and laptop on the go while my home PC is what I use at home for the most parts. Don't own other laptops/tablets so the SP3 is in that way my secondary machine, but honestly, I don't think I own anything that I'm as fond of as my Surface Pro 3. I've had it for quite a while now and I still just find myself staring at it and going like "damn, this thing is so awesome". I use it regularly as laptop, regularly as tablet, regularly as touch enabled laptop and the pen everywhere with me and the SP3, it's an integral part of the experience! Love it!
I've had mine since august last year I think, still have 26gb free SDD space. But it's true that you kinda have to think about the available storage from day one and some don't want to do that. That said, filling up 128gb isn't much harder.