MechJeb is pretty much a 'must' for me if I'm trying to put something big into space. I'm on an old laptop and during the bigger launches it's not at all unusual to get 1 frame per second or less. But I do understand the sentiment of doing things yourself instead of letting a computer do it. I do things (orbits, landings) manually for the first time (in a new version or after a break from KSP). But when I start bigger projects or start approacing "real" science instead of kerbal science, I use computer assistance. Plus, I find MechJeb to be a wonder itself. The first time I used the automatic landing on the Mun I was almost as nervous as the first time I landed manually. I couldn't believe it worked, just like that. I imagine if I looked at the source code of MechJeb, I'd just see swirling torrents of magical energy.