First and foremost, the holideck is one of the coolest lounges I have been in. Trippy British gogo styling, warm cookies, 50 different drinks, all kinds of neat stuff. Never got their internet working. Now here's the scoop from three years ago or so.

I had a PC paying membership and a plat status. I was on a KLM flight from LHR to AMS. They would not let me in with a CO plat or the PC card.

Holideck is the KLM lounge. It is the lounge that KLM puts their elites in. However, KLM is not so nice. Their skyteam responsibilities end on lounges they own. They only have Amsterdam that I am aware of. All others belong to contractors or codeshares. This means they get away with offering their passengers dozens upon dozens of other lounges, but only have to offer two in exchange. I am not a KLM fan. They don't even give PC members the same free internet they give their own passengers in their own lounges.

So anyway, that seems the logical choice.

Another thing to note - I saw a flyer for "Priority Pass" on the desk at holideck, it's a third party lounge card. It's around the same price as a PC, but it gets you a vast assortment of lounges (with other stipulations like no free guests). The PP gets you two other AMS lounges, not the KLM ones - just next door. The KLM's is still nicer but it's always packed. So anyway, I cut up my President's Club card and got in on the Priority Pass. Now I can get into an amazing number of lounges in an amazing number of cities. That is, in addition to the ST+ and the Amex But seriously, this card gets you in lounges in cities you didn't think had airports. If you bop around europe a lot, as a CO you get screwed in a lot of cities without it. You don't have to be flying any particular airline at all - you just walk in to one of the hundreds of lounges and toss it at them. I could be coming in from the United Red Carpet Club in PHL to the Northwest World Club in Detroit to the America West lounge in Vegas all in one day on this card. Flying on any carrier. They don't ask you for your ticket, just the card and your id. Totally cool. So now I don't generally care where a particular lounge is, just that there's A lounge nearby.