We purchased a guarantee cabin on Horizon to Bermuda. Has anyone purchased this and to what degree did you get upgraded?

Unless you want a specifc cabin purchasing a guarantee is the best way. The cruise lines normally sell out the highest and lowest cabins first. Once all the lower cabins have been sold as guarantees the cruise lines will then upgrade all those with guarantees (usually 1 to 2 categories) and then resell the lowest cabins again as cabin assignments at a higher fare. The slower the bookings are on a given sailing the higher the upgrades tend to be. We booked our daughter and her husband on a cruise last July in an oceanview balcony guarantee and they were upgraded to a suite(this is not the norm but they really enjoyed it).