I've actually been in contact with the developers for almost a year now begging them to bring this to bigcommerce after an awesome experience on a shopify based website. The main competitor for this product charges a ridiculous $250 or more for the same basic functionality...
Smart Search & Instant Search (searchanise) support for both bigcommerce and shopify has always been very responsive and they always follow up a few days later to make sure everything is continuing to work as expected.
If my opinion ever changes in the future you'd better believe I'll be updating my review but as of right now I couldn't be happier with Smart Search & Instant Search (better known as searchanise)!
Bottom line - If you hate money, use the other guys. If you like saving money, switch to searchanise and never look back!