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Chances are that if you are using a load balancer for TCP connections, then it does not properly handle Path MTU Discovery.

Does anybody have any field experience on how much PMTU-D actually helps?

Ours is only of the problems when it doesn't work. On the UK copy of http://news.bbc.co.uk/ they decided to play with load balancers (you may not have seen the effect of this as for the NY copy we said no). After an annoying trickle of complaints of site unreachability, which were dismissed as the net being a bit broken, we were brought in to diagnose a high profile case that couldn't be ignored (the Director General of the BBC (i.e. the boss) couldn't access the site as he went through a VPN). After a reasonably heated debate we were able almost convince them that the load balancer/PMTU-D were the problem. I say almost as instead of fixing the problem they swapped it for a different manufacturers box instead (RND replaced by Cisco). The problem isn't that they're stupid. It's hard to persuade people convinced by the marketing to buy the $20K magic 100% uptime box that it is the cause of 100% downtime for some. "Surely they wouldn't sell something that broken" and "it's your net that can't access the site that everyone else can so it is your net that's broken isn't it" are common responses. regards, brandon