I'm part way through building my first bike, and I've found you spend a lot of time looking at it, puzzling over it, reading books/watching videos on how to do each job, but each job only takes a few minutes when you actually come to do it.
I've bought the correct tools for most of the jobs, but all the tools I bought cost less than getting one bike serviced at your lbs. Most of the tools were for fitting the headset (headset press, crown race punch, star nut tool), but I also bought a quality set of cable cutters, a tool to fit the bottom bracket, and a couple of other things, I think. Everything else has just needed spanner/wrench and a multitool. (For those who haven't fitted a headset, it's easy with the correct tools, by the way.)
It's taking me a long time to get it done because life and my business keeps getting in the way, but it's pretty much done now besides finishing the wheels (which I'm also building) and making sure everything is adjusted correctly.
So, for first time around, I'd say it's easy to do, but very time consuming to figure out how to do it and what components and tools you actually need.