As well as putting you on the higher FTTC rates the premium option should give you some priority in parts of BT's network, though BT are not clear about exactly what this affects and how likely you are to see much of that effect.
I went for premium when I signed up because at the time it meant a difference between 40down/2up and 40down/10up - the main reason I was moving to FTTC was to have better upstream speeds than the ~1.5mbit I got out of ADSL2+/AnnexM so the difference was rather attractive given my line was estimated to be able to get a fair chunk of the maximum (in the end I do get the maximum).
Since upgrading to 80/20 I find long downloads from my externally hosted server (that I store large backups on amongst other things) do often hit or exceed 8mbyte/sec even for single-stream transfers (and for multi-stream transfers from multiple sources I've seen closer to 9.5MByte which is the maximum expected from a full-speed line with BT's current setup). I do sometimes have blips with youtube/iplayer/other but given the line can pull as fast as I can from elsewhere I'm guessing that is congestion further out then AAISP (and most of the time I don't see any such problems anyway). I tend to get similarly close to the maximum possible rate upstream too, though I have noticed that being a lot more variable than downstream.
It goes without saying that you check the estimates of what your line can handle: whether the upgrade is worth it to you may very much depend on that. Going from 40/10 to 80/20 is more worth while than only getting as far as, to pluck figures from the air, 50/12. I'm lucky that the line between me and the cab my home connects through is such that I can get as close to the maximum as it is possible to be with the caps BT/OR set (theoretically
a few lines could get more than 80/20 were it not for those product/configuration caps).
I'm happy to keep paying for the speed as it seems to be there when I need it, but YMMV. Generally I'd say you will only notice the difference on larger transfers and/or when you have several users actively using the connection at the same time - so if neither of those things matches your use pattern you might not want to bother. You'll not see it much at all for interactive web/mail/other use (for small transfers TCP/IP isn't up to speed before the transfer completes) or on youtube style sites (where the transfer rate limit (at these speeds) is usually going to be at their end for various reasons).
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.