Playing the Harmony H420 is an absolute pleasure. It seems like it would take a little bit of time to get completely used to the feel of it, but of course this is going to vary slightly from instrument to instrument. Its just really easy to play and to get a nice sound from it. The control knobs definitely make it easy to tweak here and there, but I don't think anyone is going to have a hard time getting a good sound from it.
I really love the sound of the Harmony H420! It's definitely it's own sound, but when it works, it works extremely well. It's perfect for emulating the sound of an upright, although of course it's not anything close to a dead match - it's just a good replacement. It has that warm and round kind of sound that just suits well in the same sort of situations. It really just depends on the type of sound you're going for, but this is a more than viable option and works perfectly to compliment something like a Jazz or P bass.
I don't have a bad thing to say about the Harmony H420, as it's really a stellar instrument. I'm a huge fan of the way this bass sounds, as I think when used in the right situations it can be as good of a sound as any bass out there. It's definitely it's own kind of sound, but it's one that I think will work well in a variety of situations. The H420 probably isn't for everyone, but if you do come across it I'd highly recommend checking it out just in case you might fall in love with it...