The Rockbox'd Clip is a nice piece of hardware in that it plays all sorts of lossless files, takes memory cards, and is so small that there's no excuse for not having it. And it sounds way better than it should for the size and price (as in, it sounds better than most normal portable players I've used over the years, including various iPods, and other models from Creative, iRiver, and Archos).
That said, there's no comparison to the Hifiman players for sound quality. The HM801 is an amazing device, with a multibit PCM1704 DAC generally only found in AC-powered equipment. I've had a number of home DAC units in the $1K-2K range over the last six years, and the HM801 sounds better than all of them, and I do use it as the DAC in my home speaker system. Bass depth and control, midrange presence, treble smoothness and detail, stereo imaging, are all vastly improved compared to the Clip.
The 601/602 (same unit, the 602 just adds the ability to act as a USB DAC) both sound much better than than any other portable player, but compared to the 801, they just fall short a bit. The bass is slightly louder, but doesn't play as deep, nor is it quite as well-controlled and punchy as it is from the 801. The 601/602 also doesn't dig up quite as much detail as the 801, so imaging suffers a bit. Being that the 601/602 uses a non-oversampling DAC chip, the overall sound signature is different than the 801, a bit more relaxed and laid-back, which some listeners might actually prefer. The 601/602 also lack the support for 24bit/96kHz high-resolution audio that the 801 has.
Anyway, the Hifiman players are quite special and unique, with the 801 in particular offering an unparalleled level of sound quality in a portable device. While it might not be the most attractive or smallest DAP on the market, it is the best sounding, by quite a margin. The Sansa Clip+ is a great device that is all the sound quality most people will ever need/want, in a very reasonable size and price. Just as it would be a bit wasteful to use $50 headphones with the $800 HM801, once you start spending $200 and more on a set of headphones, you really should pay more attention to what you're plugging them into, as it can make just as important difference in what you hear.