Really beautiful. I was here in May 2013, and the pictures on the internet don't do the garden justice - the smell was incredible, and they have the biggest bees I've possibly ever seen! Very highly recommended. Not the easiest place to get to, but here's how to do it: First, take a train to JR Yahata station. Then you have two options: 1. take local bus* no.56 and get off at Kawachi Elementary School, then walk west (the bus will drop you in the middle of nowhere, but a short walk up a slight incline brings you to Kawachi reservoir, which has some really nice views - the garden is perhaps 20 minutes' walk from there), or 2. Take the free shuttle bus from the station to ajisai no yu onsen (the shuttle bus stop is not signposted - to find it walk out of Yahata station and go left toward the bus stops, the shuttle bus stops by the little patch of grass just past where the no.56 bus stops) - the journey takes 10-15 minutes; once you've reached the onsen, cross over the road and walk up the hill - the garden is 2 minutes away (if that). Entrance fee was in the region of 1,000 yen per person. Perhaps a little steep, but well worth it.
(* Bus company beginning with 'Nishi' and ending 'tetsu' (にしてつ). Believe it or not, the profanity filter wouldn't let that through...!)