Interesting I looked up ELM and it is only for petrol cars after 2001 and diesel after 2004 so not useful for most RR P38's although for the cost it would have been good. For a RR for everyday use I would say it is important. So far on my P38 I have had 2 suspension failures and a complete lockout on the engine immobilizer in 6 months. So having the full software would be useful and save me money.
I too am contemplating getting one so am interested to see what is best for my Disco, P38 and CSK. ALthough the CSK does not need that much.
Paul you forgot Bearmach and Lynx as options.