Renewal now has several advantages, not least is that if you have less than three years before expiry, you'll be better off. If you have more than three years left you'll probably still be better off as prices are likely to rise again due to ID card implementation (over £90 is estimated, so on pure financial grounds anyone with up to 8 years to go would benefit from renewing now).
Other benefits include not having to go along to be pushed, filed, stamped, indexed, briefed, debriefed, or numbered. At some point at an unspecified date in the near future, the database for the ID cards will be activated - long before the ID cards themselves are issued in 2010.... and it's the database that's the issue.
On pure financial grounds, as well as avoiding a complex bureaucracy for as long as possible, it's worth renewing now whilst it's still fast and easy (I entered my info online, received the prefilled form within 2 days, signed and returned it with the cheque and photos, and got the passport within a week).