Don't have Archos 101 but have Archos 70 it and G tablet. Most comparison points will carry over to Archos 101...
Physical : G tablet is much thicker, with a much glossier screen. (without fingerprints) looks nicer from front, but thick from side and cheaper from back. (Archos 70 fold out stand is very nice too)
Both have 9+ hour battery life, seems G Tablet can go 12+ but I think Viewsonic has conservative power management on it.
Archos is more generic Android OS and easy market hacks available, but Froyo (2.2) updates have made the device much less stable than 2.1 it shipped with.
G tablet overlay has some interesting features like accounts for multiple family members, but is a bit clunky.
On screen keyboard on Archos 70 is terribly unresponsive, so slow that I had to install Swype (great for smaller devices) on it and used that with one finger instead of multi touch. Keyboard on G tablet is excellent (custom layout in landscape mode and handles rapid multitouch quite well).
Archos CPU/GPU, RAM, and memory layout are all passable but RAM is barely passable (256 MB) so task killer is needed quite often and internal memory for apps is extremely limited for anyone loading lots of apps (there's an app to move them, but it's a constant hassle). Don't plan on running anything beyond 2.2 or possibly 2.3 on the Archos, as it is last year's hardware config.
G Tablet, on the other hand, has about 2x the CPU/GPU horsepower, 2x RAM, 8x the internal app storage, probably good for 2.3 and 3.0.
G Tablet has some issues with apps related to 90 degree accelerometer rotation (some that use the correct API work, but lots read the device directly so you gotta lie on your side to play the games
). It also has issues with some apps because of the 1024x600 resolution (Archos 70 has the much better supported 800x480 but Archos 101 may have the resolution issue as well).
Archos devices have some issues because of on screen Android menu/home/back/search keys but at least they are visible in the dark, while G Tablet off screen keys aren't backlit so need practice in the dark, especially if you ever rotate the device.
G Tablet has *really* weird viewing angle problems, most noticeable in portrait mode in one direction. Archos 70 it is no iPad or Galaxy Tab in screen brightness, but it has decent viewing angles.
Despite G Tablet's superior CPU/GPU, even with recent significant OS improvements, a lot of things flow smoother on Archos 70it. Most actual Games/apps tend to run faster (and can keep more in memory) on G Tablet, but navigation animations and some apps like photo gallery are much smoother on the Archos.
IMHO both are fun for hacking around with, but G Tablet will be fun for hacking furthed down the road. Software on both is clearly a bit of a problem right now (hence so many custom roms or hacks crop up for them).
The 7" form factor is vastly superior for portability (fits in suit or coat pocket easiy), but 10" form factor is a bit nicer for browsing when at home.
My Archos is going back as it has had several spontaneous reboots and failed to return from standby many times (requiring 10 second power press to reboot). If it had more responsive touch screen for multi-touch typing (or proper Swype support), was more stable, and had more RAM, I'd keep it.
I've only had the G Tablet for a couple of days so the jury is still out.