What are Spelljammer campaigns actually like?

Greetings!

I'm new to Spelljammer but a somewhat longtime fan of D&D (I was introduced
to AD&D 2e but had few opportunities to play -- In the last few years, I
have had many more chances to play the current edition and my interest has
skyrocketed)

My introduction to spelljammer was, well, yesterday. I had heard of it
before, and over the past year I've had thoughts about running a space game
myself (in D20 Future) but thinking about my gaming experience I decided I
liked the feel of D&D, so I asked a friend of mine who has a lot of old 2e
material about Spelljammer and got a look at the books (and spelljammer.org).

It certainly sounds quite interesting, and I love how it expands the D&D
universe, and adds space travel but it's not just 'D&D in the future' where
technology has risen to the space age but magic is still around, rather it
provides an expansion that fits into D&D as it is, almost any setting,
including a set of internally consistant fantasy physics! Awesome!

Now to get to my point: Understanding the concepts of spelljamming helms,
wildspace, atmospheric envelopes, crystal spheres, phlogiston, and all that,
I'm wondering what a game with spelljamming as a major part is actually
like, how it's usually run.

When I picture a far-future scientific space travel game, I picture a world
with dozens, a hundred, hundreds, or more known solar systems (even if most
are just mining or research colonies). Is Spelljammer often run in such a
world, or are there usually just a handful or even a single crystal sphere
involved, and everything happens in that little space? (Admittedly, in any
setting other than space I wouldn't call one or more solar systems 'little'
-- I guess part of it is that for most systems, worlds beyond the main
civilized world aren't very detailed so their size is very small in my mind)

I mean, I can picture a game where other planets, asteroids, and moons are
just more locations for dungeons or places for enemies and allies to come
from, but most of the action is really centered around the main world. I can
also picture a game where, like a fantasy Star Trek, the party sails through
the phlogiston visiting all sorts of crystal spheres, in fact rarely
revisiting the same one twice. I can sort of picture a game where the party
aren't really on some grand quest, just out for money and themselves, and
the adventures consist of the various jobs they take.

I guess all of these have non-space analogues, really -- the ocean makes a
good analogue for space if most of the planet is unexplored by the PC's
civilization.

What I'm asking here is, what sort of Spelljammer games have people run, and
can space travel make possible new kinds of adventures that don't really
have parallels in groundling games?