As I mentioned earlier though, the Shures do lean more towards clarity at the top end, while the Custom-3s give you a bit more bass response. That's not to say that the SE420s really lack bass, or that the Custom-3s lack high end clarity, and if you listened to each in isolation, you'd no doubt be happy with the breadth of either sound stage. If you absolutely forced me to make a personal choice, I'd probably go for the SE420s, since my preference tends to lean towards absolute clarity, without losing too much of the low end - which is probably why I favour the triple driver Shure E500PTHs as my personal choice of earphone.
However, I have a sneaking feeling that much of the buying public will prefer the slightly fuller, ever so slightly bass heavy sound of the Custom-3s, and I have a suspicion that Klipsch believes the same thing. Couple that with the lightweight, extremely comfortable design that Klipsch has created, and you've got a pretty compelling package. I think Klipsch has developed a range of earphones that will suit a broad spectrum of listeners, looking for a serious upgrade to the bundled rubbish that came with their player. And even though I may (just slightly) prefer the sound of the Shure SE420s, I think that I'd probably find myself in the minority.
Despite being larger and heavier than the Images, the Custom-3s are still very light and comfortable in your ear. More importantly though, the dual-driver design makes for a broader sound, with more impact at the low end than the Images could manage.
The Custom-3s are going head to head with Shure's SE420s in terms of both price and specification, and the sound produced from both sets is mighty impressive. If you favour high end clarity in your music, you're probably better off going for the Shures. But if you're looking for a rich, full and powerful sound, across a broad range of musical styles, the Klipsch Custom-3s will suit you down to the ground.