The pros/cons as I see them (some, all, or none of which may be significant to you):
Con: throttling/shaping of some traffic at busy times.
Mitigating factor: if you don't use P2P or a VPN (which may be mistaken as encrypted P2P traffic), you might never experience the effects of this shaping.
Con: I've heard a fair few stories of problems with transferring to BT+FTTC to another ISP
Con: I've heard many many horror stories about BT's after-sales support.
Pro: Lower cost, for setup certainly and depending on your use patter ongoing too (an 80Mbit connection could rack up an expensive bill in peak time!)
Con: Explicitly paying for bandwidth used (not "unlimited")
Pro: No traffic shaping at all (aside from the basic QoS priorities needed for IP networks to run generally smoothly, which you'll find done on all networks)
Con: Signup/regrade charge
Pro: I've heard very little by way of problems switching to AAISP+FTTC either from their other product lines or from other ISPs
Pro: I've heard very little by way of problems with their after sales support (when needed which isn't often as they have a reputation for providing a very reliable service). They appear to be very good at chasing BT/OR on you behalf when there is a problem that is in their jurisdiction, and very good at keeping you informed of progress when something is being investigated
Pro: Static IP address should you need that, multiple IPv4 addresses too if you need those
Pro: IPv6 fully supported if that is of interest to you, with a decent address range allocated by default
Current Line: Andrews & Arnold (AAISP) via 80/20 FTTC, getting close to the full rate both ways, joined July 2011, upgraded from 40/10 to 80/20 May 2012.
Previous setup: Be Pro with UploadPlus (ADSL2+, AnnexM), 12ish Mbit down, 1.6 up, happy customer for ~2.5 years.