Imho they're worthless, and the registry can revoke any that were bought on the aftermarket, or indeed any that are not held by licenced professionals.
They are working out dandy for me. I sold Switch.pro for $2,000 in November to a Japanese engineer, see developed site and dnsaleprice.com, 2 weeks back I sold Booking.pro for 5,000 Euros to Booking.com. WHOIS is still showing my name but the sale completed a week ago and I got my money last Friday. I have received enquiries for half a dozen other .pros in the last 3-4 weeks.
I bought Booking.pro from the OP for $2,000 in Sep 07, as far as I know he registered it for $99 in May 07, not a bad return in 5 months? The OP also sold Tour.pro, Claim.pro, Claims.pro, Freelance.pro and Event.pro for low to mid $X,XXX, he hand regged the lot in 2007. He must have cleared about $15,000-$20,000.
I have also been adding .pros, for example I bought Game.pro for $5,000 in Sep 09 so that was another nice .pro sale. Pro is a very useful branding suffix, do a search on any trademark database to back that up.
It's untrue that the .pro registry can revoke .pros bought on the aftermarket. The Japanese lighting engineer who bought Switch.pro transferring it in to a new Encirca.com account, he could updated all the WHOIS details apart from the registrant details, RegistryPro sent him a verification link, he followed the link and filled in his professional details, within 24 hours he was verified and able to change the registrant details. You have to be qualified to own a .pro or at least have the nous to register a company for £29.99 and put the .pros in the company name.
On the upside, professionals usually have more money than non-professionals so if they are looking to develop a website they are willing to spend mid $X,XXX on a decent .pro generic keyword. Unlike unqualified domainers and minisite developers who want your .com off you for low $XXX.
Ignorance is bliss. I caught Politics.pro, Stockmarket.pro, Local.pro, Coupons.pro, and other great single word generics on Snapnames in 2009. Not making any money net, but covering my renewal fees on 350 top .pros. 2010 could see a complete liberalisation of .pro and with the quality and depth of my .pro portfolio, I would be selling a Switch.pro and Booking every month. Domaining is about spotting opportunities before everybody else spots them. The OP spotted domainer fatigue in .pro in mid 2007 and made a twentyfold return on reg fees. 2 years later and the average poster on this thread is still clueless about eligibility rules and registy policy, by the time .pro is sold on Godaddy and every man and his dog can register one, there won't be any decent keywords left.