I upgraded on day one of iOS 8.0, mainly because I am an earlier adopter, and while performance dropped, for me the features made it worthwhile. 8.1, 8.2 and 8.3 have gradually improved things but I feel it still feels a bit slower than 7.1.2.
To answer your question, if you need the new features/ security updates and you can put up with a bit of degraded performance, and general iOS 8 instability (though 8.3 has fixed this a bit) then go ahead. If not probably don't.