Against:
1. The small size is probably too small to be much use
2. The larger size is still small and can be difficult to fold up and put away
3. Very expensive for what they are
4. The optional extra train is probably a bit of a waste of time simply because it isn't illuminated and so needs lighting separately - and if you have the space and skills to do that then you probably don't need the Hi-lite in the first place.
For:
1. They do exactly what they claim to do, i.e they allow a white background (nothing to do with high key BTW) to be produced with a minimum of skill and a minimum of space.
2. They can be used with any studio flash heads.
3. They have been almost universally adopted by event photographers, for whom time and space are in short supply, largely because they are so easy and quick to use.
Having said that, if you have enough space you can get exactly the same effect, on any size background you want, with a couple of flash heads and a plain background.