I own the iAudio X5L (and love it) so I'll answer some of these, though most of your questions about the X5 are answered on these pages:
http://onlinestore.cowonamerica.com/index....ATS&Category=60QUOTE
First of all, let's start with the iAudio X5.
1. Does the player support gapless playback with any of the supported formats?
No.
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2. How is the Ogg Vorbis support - any bitrate limitations like with older iRivers?
It supports up to q-10, and a low-end isn't listed for it, and I've heard from people who've used as low as q-2 on it.
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3. Are all FLAC compression levels supported, only only up to 2?
It only supports FLAC compression levels 0 through 2, but frankly I don't really know why anyone would want to use FLAC on a portable player anyways, heh.
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4. Do videos play with a decent quality or do they stutter (I am asking because the product page reads "It's possible to convert various video files to MPEG4 up to 15 FPS using JetAudio.")?
I've never experienced any stuttering of video on mine, but yes it can only do up to 15 FPS, and you can convert them at different quality levels to take up more or less space/have better or worse quality, and the conversion process is quick.
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5. How is the battery life?
If you get the X5L model it's unbeatable! Up to 35 hours of MP3 playback. If you get the regular X5 model it's 14 hours though, which is why I got the X5L since it's not that much of a difference in price, and I've yet to use up all the battery life on it yet, even with long periods of Vorbis and video playback.
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6. Is the recording function good, or does it skip like with some iRiver models?
I've yet to experience skipping with the recording on mine, though I haven't used that function a whole lot.
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7. Is the radio sensitive enough or does it introduce noise often?
Never used the radio function on it so I can't answer this one.
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8. Anything else you can say about the player?
Umm, well, it's great! The sound quality is fantastic and with the X5L model you can't beat the battery life.
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Finally, which of the two players would you recommend?
Well I've actually encountered at least one of the video iPod models from a co-worker, and I can say that it's much thinner and lighter than the X5L (though the X5L is also a little fatter than the X5 model, because of the battery I'm assuming), and the video quality on the iPod is better than the X5 (the framerate isn't limited to 15 FPS, the screen's a little bigger, and hey it did come out later than the X5 afterall). So if the video quality is really important to you, you'd be better off going with the iPod. Personally though I love the X5L and doubt I'll get any other brand of MP3 player when I upgrade. I use Vorbis for my music so that was a big part of my decision to get it, along with the battery life, but the sound quality is great too and if you like EQ features the BBE effect sounds great.