Geo IP + Search Engine Crawlers

It all depends on the volume of traffic you have generally. Irules impose some overhead, but if you're not maxing out your device, then this should not be a problem. You should consider doing this using Local Traffic policies if you can express the logic, as those are more efficient than irules.

In any case, user-agent strings can be easily forged, so as long as you're not doing anything security-critical, then it sounds reasonable to key off this. If you're doing anything that relies on the integrity of this header, then you might want to work out a way to pair the user-agent strings with valid source IP addresses obtained via DNS, but this adds the challenge of managing the IP to user-agent mappings and keeping them up to date.

My two-pence.