I can't really call myself a frequent flyer but for what they are worth here are my views on the subject. Having visited ACs about ten times since making Plat I purchased membership shortly after 9/11
The ACs I visit often feel more crowded than the airport because they are in an enclosed space.
The coffee is awful.
Finding a plug for the laptop away from the workstations is difficult
The endless television is annoying.
It's full of people I dislike travelling with, namely professional travellers.
However, for $300 or less I think the money is worth it specially when you consider what other types of expenditure you make during the year which exceed $300 and are even more momentary. My reasons for being a member are:
It is another option at the airport, sometimes I am not in the mood to be in among the masses and sometimes I am.
The front desk staff and check in desk staff are probably the only people you will meet at the airport who will smile at you.
I suffer from SAD and so like the luggage tags.
Twiglets at LHR
Note: The only ACs I have been to are LHR, ATL and ORD which are all on my commute.