Pi vs Odroid
I have both, and yes, the U3 is a really excellent board. Much better value if you need the extra CPU (quad core,, 1.7GHz) and RAM (2G). The various Odroids are about an order of magnitude better than the Pi on this score. They've also got a really good and active support forum and it's quite an open platform (apart, mainly, from the MALI graphics drivers). They also support running Android on it, if that's your cup of tea.
The downsides, as I see them:
* very limited warranty (two weeks; though if you buy from Pollin.de, the European reseller, I guess that you might get the full EU-mandated warranty period)
* cost of shipping from Korea (though again, ordering from Pollin.de can cancel this; RMA also expensive if ordered from Korea, since you have to pay return postage if the board turns out not to be faulty)
* slightly less open than Pi (though it may be a wash due to proprietary Broadcom stuff existing too)
* awkward/inferior OpenGL/ES implementation (Pi's graphics hardware is frankly amazing at the price and will out-perform machines that cost multiples of the price; odroid's xbmc is getting better, though)
* quite a bit more expensive if you opt for emmc memory instead of SD (but you don't need to)
* U3 dropped the headphone output that was on U2 (though original Pi's sound output is only ~11 bits, so again, probably a draw)
* few GPIO (though there are add-on boards to compensate for this)
I think it's a case of horses for courses. Both boards are cheap enough that many people will be able to buy both and see what works best for what task. I find Pis to be great for media applications (music, video display and transcoding), as a NAS (with external USB disk), APT cache proxy and other always-on web-based applications that aren't too demanding on the CPU. Also for messing around with GPIO, either directly or with a Gertduino. You might be surprised how many things a Pi is good enough for, despite its lack of raw power.