I use them all the time. They don't cause a sugar spike because they are mainly complex carbs (maltodextrin). If you've had a good breakfast, you don't need one to start. I use one every hour or so on a long ride, then wash it down with water. Works wonders.
I bought a couple of these bottles and load them up with 4 gu's and top it off with water, so it comes out easier. I don't do long rides without them. I usually don't use them on anything less than 40 miles.