I won't waste your time explaining the advantages or disadvantages of the concept, any reasonably intelligent person can get there on their own and there is already much information (and marketing) floating around. What I will go over as thoroughly as I can are all the commercial options available right now, generally the conversation arises in either an ultralight backpacking context or a tactical/preparedness context and something usually gets left out.
I have only included the fill wight of the synthetic options because I find it more useful then temp ratings but I don't have as firm a frame of reference on down fill weights. I have tried to include only options that can be used as a full length sleep system, hence my omission of the Kifaru Woobie Express and the horrible Sierra Designs Elephant Foot.
Keep in mind that temp ratings on top quilts and open blanket style sleep systems are even more variable then sleeping bags (which while wrong are reasonably consistent).
I can't tell you which came first but the Jacks 'R' Better sierra sniveler is the earliest I know of:
firmly in the lightweight camp, a down top quilt with liberal use of velcro. Optional hood and sleeves add a lot of versatility. A slightly tapered flat quilt with a velcro closing head hole, worn poncho style. Certainly the most well known in ultralighter and backpacker circles.
optional hood: 2oz, $60
optional sleeves: 5oz, $80
Another of the early contenders, the Exped Dreamwalker (previously the Wallcreeper):
A mid-weight, feature heavy sleeping bag/parka. Has armholes, front pockets, a jacket style hood and a full zip front with a bottom cinch cord. The bottom is un-cinched and re-cinched around the users waist for walking mode. Can be opened flat for a spacious quilt as well. I've heard mixed reviews of how well the foot cinch works to seal out the cold. Does have baffles blocking the armholes.
Available in two weights of down and a synthetic version.
It deserves special mention that this bag has a matching bivvy sack/rain jacket, but the idea of a "rain vest" holds little interest to me.
Exped more recently offered a second wearable sleeping option, the Dreamwalker DUO:
New kid on the block, the Hill People Gear Mountain Serape:
square poncho sized blanket with a clever zipper system that wrangles the excess fabric better then a typical toggle or belt. Attached hood with a 1/4 zipper opening gives it a pretty big wearability advantage over it's competition. Uses Primaloft insulation unlike the bulk of the poncho liners out there, what I've read/heard seems to imply that climashield has better warmth to weight but Primaloft deals with water better. I think in general this product has introduced the idea to a lot of users that otherwise wouldn't have thought twice about a "silly ultralighter" product like the JRB quilt. Better marketing and customer interaction has made this a more popular option then some of the similar products.
also usable as a half-zip sleeping bag:
40*? (general consensus from Mt. Serape users, seems very optimistic)
Probably the most sleeping-baggy of the group, the previously mentioned Sierra Designs Mobile Mummy:
Perhaps the least intuitive as well, but no less clever. You unzip the bottom a ways and stick your legs out while you button the bottom out of the way behind you (seen below center):
Very similar to the Exped but without the hole at the bottom and with a more nuanced shape and design. No zippers blocking the armholes, relies on wide overlapping baffles instead.
The Ecotat was an earlier option but I knew it had been more or less discontinued. I recently found out it is still being made by Wiggy's as the Freedom shelter:
very little information on their website about it, but I think it safe to assume lamilite is involved and it is on the heavier end of things. Built like an Exped Dreamwalker but with no armholes and no pockets. instead it has a head hole to wear it as a poncho.
A lesser known option, Wiggy's Insulated Poncho:
By far the most affordable option, just a quilted poncho liner with attached hood. Despite the comparatively hefty fill weight word is that the stitch-through quilting inhibits its warmth. Only other thing I've ever heard about it is that it does not pack down very well. Uses lamilite and climashield insulation in an unknown combination. Available in MARPAT which is hard to find these days.
A higher end version of the same, the 782 Gear Smoke Kloke:
Perhaps the least known option I've found, the Integral Designs Poncho Liner:
I've never heard anyone mention, recommend, or otherwise acknowledge this product, I found it just poking around on their site. Uses Primaloft Sport insulation, has a silly half-circle zipper head hole and a perimeter zipper for sleeping bag use.
$220 (listed elsewhere on their sight for $150)
And finally, last and in my mind least; the Poler Napsack:
A shameless rip off of the Exped Dreamwalker, (I really hope there is some licensing agreement no one has bothered to mention) if you handle one in person you can feel the cheapness radiating out of it.
I include it only out of a determination for completeness.
I'm simply shocked I forgot to include this option, I've know about it for some time but it just slipped my mind:
Feathered Friends Rock Wren:
(photo from Outdoor Gear Lab)a very light, high quality option from a reputable dealer. It functions like a cross between the Mobile Mummy and the Dreamwalker, but could be older then both. Unfortunately this bag has gotten critisized for poor hood seal and no baffles over the arm zippers. A warmer version is available as the "Winter Wren."
So what are my thoughts about all these? The Sierra designs is the only one I'd really consider for my only sleep insulation. The JRB might work but the verdict is still out for me on top quilts.
I am mostly interested in one of these as an emergency layering/sleeping piece (coupled with a poncho in a car bag or day pack) and as an overbag for a normal sleeping bag.
For this purpose the HPG Mountain Serape definitely seems to offer the most versatility and value, but I would change a few things about it. Since it is made by and for larger guys I could get away with a considerably narrower width and would trade that weight to bump it up to 3oz or 4oz of insulation.
What I will probably do is buy the Wiggy's poncho just because of the price and cut and sew on it till I get a better idea of what works for me. If I love the system I will probably upgrade to the HPG.
If you know of an option I forgot or have never heard of please point it out and I will add it to the line-up.