I've been playing, but I stopped recently. Game just takes way too much time.
There isn't really any good way to defend yourself while offline. The AI is too stupid to defend in your absence with ships, and even with really heavy facility defenses a player can easily get around them. You can make it harder to attack you, but unless you can log in and start leading the defense you'll lose.
For folks who can play a lot and log in at odd times after they get email alerts of being attacked the game is great. But I don't want to sit around near a computer all day waiting to log in and fight at random times during the day.
If the AI was good enough for units to put up a reasonable defense while your offline it would be great, but otherwise... meh.
A few AI fail examples:
Missile fighters attacking ships.
Under player control: fly by at 150+ and bomb the crap out of the ships every 30 seconds and stay away for the rest of the time, never take damage vs most ships and only some damage against targets specialized anti-fighter, and staying with the ships no matter where they try to run due to maintaining radar contact.
Under AI control set to maneuver: fly up to the target, launch missiles, and fly around at 10 or so. Take massive damage from everything, even ships without anti-fighter guns. Follow ships only if they're in visible range, ignoring radar contacts.
Bombers attack your planet:
Under player control: launch ships into orbit if you have free CP and attack the bombers as a group.
Under AI control and set to attack orbit: Launch and the ones who happen to get into visible range while going to orbit attack. Have a bunch of these set up on multiple planets? Enemy can send one ship to each one and the AI will launch all of them regardless of CP, then they'll kill eachother instead of the enemy due to going over CP limit, so can only really defend a few planets from bombs - and only if all ships set to go to orbit together are within CP limit.
Being attacked from ships out of range:
Under player control: Fly toward the target and engage, or run away.
Under AI control: sit there and die.
It could be a fun game if you had people on 24/7 and all guilded so you'd have someone around all the time to defend your assets, though.