For what it's worth, I still find the HP18 a great dram.
What has to be considered is that everything is relative - a not so good experience with HP18 can only really to be guaged against itself. Most folk who have found HP18 to be lacking recently have probably been wow-ed first time round and find the second/third or whatever experience to be a bit of an anti-climactical - perhaps a case of over familiarity? This, however, doesn't mean that the whisky is a lesser quality.
Personally I'd risk going with the 18yo - its one I'd take to a desert island, along with Led Zep IV, Wind in the Willows and Mrs C (
loads of brownie points there
) and ma dug - and home, garden................
Anyway - I'm not going to tell you it's the best thing you'll taste. It's like telling someone they'll find a film funny - they probably won't!. Everyone had differing tastes. What I will say is that HP18 it is a well respected whisky and liked by many people. Jim Murray described it along the lines of opening a honey pot that once held peat embers
Choochoo gives good advice - you could try the 12 first and if you find it pleasing, give the 18 a shot.
Let us know how you get on.
I understand there is more of an appreciative taste for them in GB. Here in the US, we have no sophistication, elegance or patience for such a taste, and I am no exception 
, that HP 18 is a great whisky. But I found that it seems to deterioate in an opened bottle quicker than most. Both bottles I've had have done it.
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