Since I don't own one, you have to take this as a generalization.

Any HF mobile antenna is going to be a compromise, just won't be as
'good' as a real-live full sized antenna. If that same mobile HF
antenna is used as a base, it's still gonna be a compromise. Of
course, if you don't have the room for that real-live full sized
antenna, then that little one is certainly a better option than no
antenna at all.

The cost of this antenna relates more to convenience than anything
else. Just how much is convenience worth to you? Being naturally
cheap, I can change a lot of 'taps' for that amount of money, so I'm
not likely to own one very soon (after I win the lottery!). I don't
have any deed restrictions, and quite frankly, I couldn't care less
what the neighbors think about antennas... within reason.

The VHF antenna and the HF antenna. As previously said, distance
between the two makes for 'better'. What's the 'best' distance? A
nice answer is, as much as possible. That's the same for any two (or
more) antennas not just HF/VHF. Along with not having a 'zap' happen,
it's also nice if the radiation/reception patterns are disturbed as
little as possible.

Great answer, huh? Lot's of specifics, CYAs, and so on. Oh well...
- 'Doc