I love it. I have a couple similar projects I've worked on in the past (a couple of which were what got me into engineering in the first place) and the clarity of vision you can achieve when you're really truly building something just for yourself is unparalleled. What follows is also a very unique sense of fulfillment; as you've eliminated all the societal contributors to the sense of fulfillment (which are of course fickle), what you're left with is something that by definition had to be made to make you and only you happy, and it lasts over time in a way that is durable and pure.

My greatest regret these days is how often it feels like I lack the time to do such projects -- but that of course is a cop out on my end! The hard part is only getting started and being consistent; you don't need to do that much on a week to week basis to get to somewhere really meaningful after a few years.