It clings here on there so you're constantly shuffling around inside it and can catch on twigs, you're tripod and all sorts meaning more adjusting still (hardly useful for trying to keep birds in front of you) and it's hard to keep tabs on whats going on around you and then use the camera quickly. It does have the big advantge though of offering complete concealment and being incredibly portable when you roll it up, can fit quite nicely into a bag. I know someone who manages to use one to great effect so there not totally useless but requires patience (something which I struggle with) to be used to best effect. TBH if space or other factors stop you from using a proper pop-up hide I think one of those chair hides is the next best thing.
Neither will ever be as good as a decent pop-up hide which gives you complete concealment, plenty of room for you, your tripod and any other bits you may want (like food and drink) and with the ability to move freely within it if you need to stretch a little. Only a pop-up hide gives you these things and is the only real solution for long vigils. And of course you can leave a hide out there overnight or even for a week for that matter to let birds get used to it.
The basic bag hide design could work but it needs tweaks. I think it would work a lot better if there was no hood and was open at the back as this would allow you to just slip underneath with ease if you see something coming. In fact with mine I did actually cut it open at the back with scissors but a slight hitch was I was using it at the time and didn't exactly make a good job of it, it has rather too much material on one side now! :'D
