I always get a kick out of discovering a haiku that I love and THEN realising (or sometimes not until it's pointed out to me!) that it's 5-7-5.
The worst ones are obvious, the great ones aren't - not a spare word contained therein.
Personally, I write "free-form" haiku and on the odd occasion when I have tried 5-7-5 have found it extremely tough to do anything decent, so hats off to those who write well within the confines of a syllable count.
I don't count syllables at all, but judge the poem's rhythm and pacing on its sound when I read it aloud.
Which is a good tip for new writers - read your poem aloud. If the tongue trips over a word or a pair of words, then the eye is likely to as well. If you read aloud (even under your breath is good if you're surrounded by non-poets) you will quickly "hear" the edits that need to be made.