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I have a Hoya variable ND and I like it a lot. It is sharp, thin profiled, and doesn't have much color cast. It's also quite a bit less expensive than the singh ray or Heliopan.
It's the best vari-ND i've tested. However you should be aware that variable NDs and wide angle lenses don't mix well. Variable NDs create X patterns near the end of the their darkest range, which show up clearly on wide lenses. The problem goes away around 50mm (on full frame), but at 24mm or wider you won't be able to take advantage of the full range of the filter.
For wide glass I just stick to normal NDs.