I have the Gigabeat G5. I got it from Japan back in summer. It was my first HD based player and i have to say it was really impressive. I gave a few friends with the iPod a listen and they said the sound was better than the iPod. Also, if you have seen the pictures from Google, etc it is thinner than the iPod (3mm thinner

). It is wider in size but shorter in height. It is also about 20 grams lighter.
Here's an extensive review on it, but i'll continue ranting. It is a very comprehensive review.
It is definitely a competitor against the iPod, but i don't think it can compare to the iRiver H series, but the newer line of Gigabeats (the F series) would definitely had been my choice if it were actually a choice back then and
would be in competition with the iRivers. Those had recently come out with a colour screen, etc and is even more expensive.
There are 3 sizes for the Gigabeat and many colour choices. There's the G5 with 5GB (mine), G20 with 20GB, and the G40 with 40GB. Only the G20 and G40 come with a cradle, but i'm sure you could get the accessory for the G5 if you are in Japan at the time, if not, you can probably get it online (I'm awaiting it's coming to stock on a
site currently). The cradle bunches in an amazing feature which I would really like to have. It is capable of acting as a HD on a wireless (or wired) network through a seperate wireless (or wired) adapter.
I have to say, the sound is simply exceptional once I heard it and imo was better than the iPod's (the Gigabeat also looks better

). It sounded even better with the long list of EQ presets. The blue backlight also looks radiant, sleek, and elegant on the black sides. Battery life is stated as 11 hours and I would trust that without any EQ settings and no song switching would actually be 11 hours. With a full time use to no battery, i have used about 9 or 10 hours as I use an EQ setting and set volume so that outside sounds arent too audible as well as switching songs every now and then.
Stock earphones aren't amazing and definitely dont look sharp, but i got a deal in Japan. I could get a pair of any head/earphones on their shelves and have 5000 yen off (around $50CAD). Also, the stock remote is small and not prone to fingerprints and packs all the necessary buttons in there.
After using it for more than half a year already, I have to say it has to be the best player in its market and price range I could get. I have never had any problems with it except one time where it said there was some HD problem (in Japanese) but after letting it rest with the battery off fixed it. Also, if you do get it, it took me a google search to figure out how to change the language back to English (I can tell you how to get to it if you get it). Lastly, the software it uses is straightforward and not bloated and modifies the audio file with something of their own to prevent copying audio files to another computer. It supports WMA as well which the iPod does not. The Gigabeat also acts as a hard drive in Explorer for files.
I hope that helped. Btw, when i got it, i specifically wanted to think non-conformist, so Gigabeat i went. I didn't want to be another iPod user