We had a trampoline when my older kids were young and they did really enjoy it - also made them very popular with the neighbor kids. As the others mentioned, be sure you have very strict rules and proper insurance coverage. I asked for a signed permission slip from neighbor kid's parents releasing us from liability - prob wouldn't really hold in court, but did it anyway.
When we weren't home, my husband would put it up on its side against the house so it wasn't so tempting for kids to sneak in and bounce.
Be aware you'll have to supervise very closely - our rule was only one kid could bounce at a time. Otherwise they will inevitably bump heads, chip teeth etc. Also, the heavier child will cause the lighter child to bounce way higher, even with an enclosure, if they bounce too high, losing control they land wrong and can be hurt.
Also - my son chipped half his tooth bouncing on a trampoline (at someone else's house) since he thought it would be fun to bounce a bowling ball next to him while he jumped! Obviously there was no supervision happening when this occurred...So - no other items should be allowed on the trampoline while anyone is jumping as they can bounce up and cause injury.
You'll want to re-position it every few days so your grass doesn't die underneath.
That being said, it's great excercise - lots of fun and no mess! - for grown up kids too :-)
have fun!