The boot-up time is one of the major issues for me, too. I often play/pause my music or audiobooks, so it is a big waste of time waiting 30 seconds for a boot and then to press play. Some players will boot directly to play.
The Muvo2 will boot up in less than 4 seconds unless you haven't used it in a day or so (kind of like a sleep mode). The Nano boots instantly into playing if you have been using it recently. I REALLY like that, but, of course, that uses battery time to keep up the sleep mode. The Rio Carbon takes up to 10 seconds to boot and you have to press play after it boots.
BTW, if you can get a Muvo2 FM 5gb along with its included remote, you can just pocket the Muvo2 FM and change tracks and volume from the remote clipped to your pocket. I bought a Muvo2 FM shell from Ebay and added the drive and battery and remote (also from Ebay). The FM remote seems more rugged than the FM wired remote of the Muvo2/NJB3. I went through four of those remotes (cord flexing breakage problems). If I need to re-charge the power, I either swap batteries or plug in an Ipaq battery pack charger.
When I want to use my Rio Carbon, I keep it in a small cloth bag and clip the bag to my shirt and stuff the Carbon barely inside my shirt. I can then feel and press the D-pad control as needed and twirl the volume wheel. To charge the Carbon on the go, you can use a battery pack USB charger.
With the Nano, I use the new Apple Radio Remote and clip that to my shirt and control the tracks and volume. The Nano goes into my shirt pocket behind my nerdy pocket protector with pens, box cutter, pen-like screwdriver combo, laser pointer, lens brush, penlight, etc. There are some USB battery pack chargers for Ipods but I don't know how reliable they are. For portable use, I have a Powerjolt auto charger which I plug into a 12 volt cellphone battery pack.
Pros and cons:
Muvo2 pros: Rugged, easily disassembled for repair and replacement, replaceable inexpensive battery, fast bootup, accelerating FF/RW, standard 5 volt DC jack, standard USB jack, UMS support, multi-band equalizer, navigation by folders, (FM version - FM radio, FM recording, voice recorder, simple remote).
Muvo2 cons: box-like design, small dim screen, simple Muvo interface, no clock, no Audible.com support, no id-tag navigation, no on-the-fly playlist support, no sleep timer, no bookmarking.
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Carbon pros: slim design, easy to operate controls by sight and touch, long battery life (20 hours or more), Audible.com support, large bright screen, very informative screens, accelerating FF/RW, multi-band equalizer, USB charging, UMS support, navigation by folders and by id-tags, on-the-fly playlist support, sleep timer, bookmarking, low pricing on Ebay, voice recorder.
Carbon cons: don't drop Carbon on volume wheel, ten second bootup time, very hard to disassemble, no 3rd party replacement batteries currently, no radio, no optional remote.
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Nano pros: Itunes, very slim and small, very rugged, very bright large screen, great user interface and controls, great looking, lots of accessories including replacement batteries, Audible.com support, scroll wheel support for quick access to progress bar, navigation by id-tags, on-the-fly playlist support, sleep timer, bookmarking (by Itunes), very fast transfers, optional radio and remote, flexible docking port usage.
Nano cons: Itunes, no music UMS support, expensive, scratchable, must use hold switch when pocketed, no voice recorder, proprietary port.
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I have had my Nano for only a week (actually had to exchange it because of static during charging on the right channel), so my views on it may change. The Carbon has been my constant companion along with the Iaudio 5.
The Iaudio 5 would be my main player IF it had Audible.com support and it had 4gb of storage. The I5 is rugged and easy to operate clipped to my shirt pocket. Moreover, you can memorize the menu structure and operate the I5 blindly.
Cowon is releasing an Iaudio 6 with a miniHD but the controls are different. The new Sandisk E200 6gb flash player looks interesting and it will probably support Audible.com (other Sandisk players do). It comes with a replaceable battery and a color screen and is Nano-like in size.
It's a tough choice. That is why I have all three and am keeping them. I probably didn't need to get the Ipod Nano but the bug bit me.
I hope this is enough information for you to make a decision.