As a very satisfied long-time lightsphere user (since it first came out), I have had no problems with taking it with me.
Why? ... well while it can take up a lens-slot in my backpack bag, I can put other things IN it, including one of the several lenses I use, or extra batteries and charger — virtually any kind of smal junk I want to corral and keep easily available. It only takes seconds to pull it out (dumping anything that may be in it into the area I just pulled it from), and just a bit longer to restore it.
I lose no storage space in my bag, and the lightsphere's dome slips in almost anywhere (and can also surround something small if needs be).
Do I take it with me all the time. No, not unless I am anticipating using one or more of the three flashes I often use together. And yes, I do have some fold-away and self-made light modifiers I also use from time to time. But for almost instant no-fumble mounting, outstanding results, and transparant usage (other than looking like I have a gelatin mold on my flash) I personally find it a very usable tool befitting of many lighting situations.