Glenn,
My 2p-worth:
Must have:
Cruise control (depends on how much M-way driving you do)
Xenons with DRLs (really make the A5 stand out on the road)
Hill Hold Assist (excellent for the price)
Parking+ (ie front and rear sensors - it's very difficult to judge the long nose on the A5!)
B+O sound system (just awesome!)
Heated seats - fantastic in the winter, and also loved by my wife and her bad back!
Worth considering:
19" Y-design alloys (I think even 18" wheels look small on this car

)
Interior light pack (particularly useful having puddle lights for entry at night)
Storage pack (I'm going to add the shopping bag hooks to my boot as they look excellent - I'd also like the netted side cubby, but it looks a bit expensive)
6 CD Changer - worked well in my old S4, but now I use almost exclusively iPod through the AMI)
Bluetooth - probably depends how wired-in to your phone you are. I just turn mine off or silent when I'm driving and let the voice mail take a message - if they don't leave one, it couldn't have been worth the call in the first place!
Adaptive lights - excellent if you drive out in the country much - if not then drop down to not worth it!
Probably not worth the money:
Any inlays other than standard - the hologram ones aren't that different to standard to my eye.
Valcona leather - the standard Milano is just fine really.
Tyre pressure monitor - use a gauge or just check your tyres regularly.
Satnav+ (sadly way too expensive for what you get - if I was ordering agian, I'd probably not bother and get a Dension system for my car, saving around £2000!)
Auto wipers - can't stand them myself as they're completely unpredictable. I wish there was a way to turn off the sensor and get manual control back!