I have mainly used my bivy's in the winter for mountaineering, but have used one while mountain biking. I tested it out at a campground and didn't find it too bad, but wouldn't want to spend a day inside in the rain. You can easily read a book or look at a map inside it so it does have more room than a regular hooped bivy.
I have four bivy sacks, a North Face military camo Gore-Tex (GTX) for stealth sleeping and a GTX Outdoor Research one I usually lend to friends that come along, but both are pretty much just waterproof sleeping bag liners. I also have an
Outdoor Research Advanced Bivy, but rarely use it since I found one I like much better. It's the
Nemo GoGo, and it's sort of a cross between a bivy and an ultralight tent. There is more room inside it than the other bivy's, but not as much as a tent. It does have a screen entrance for those buggy nights and a storm flap, but if you're looking for no condensation when it's raining outside, you will be disappointed since it's just a single wall with little ventilation when buttoned up. The unique thing about it, however, is there are no poles. Instead it uses an air bladder that is easily blown up in seconds, and it's surprisingly rigid in the wind. The bladder itself is pretty tough (I always carry a spare but have never used one) but it's also sheathed inside a second skin, so it's protected against abrasions. It's very easy to change out due to the velcro system holding it in place. I stake it out with 5 stakes (this is mainly so there is more room inside) and inflate it, and it'll be up in literally under 60 seconds, ready to crawl into.
Here is a picture of it (this is not mine, but is identical, I can't find the picture of mine at a the moment and I ride a Salsa) and it weighs about 26 ounces, but my Advanced Bivy weighs just over 32 ounces (yes, I am a hiking weight weenie).