Using the trial right now—I think that the interface is amazing, in comparison to other font-editing software. (I still think FontLab is the best when it comes to handling merging contours, but Glyphs is by far the most intuitive for most tasks.)
However, as others have mentioned, the price is a sticking point: $300? In 2011? Yes, FontLab and Fontographer are more expensive, but times have changed, and software pricing reflects that. This price would have been considered "cheap" in 1991, but in 2011, it's a bit steep. No, he probably shouldn't charge $2.99, but $100 might be a better price.
Another issue is some of the text in the interface and in the help -- it's obvious that the copy is written by non-native speakers of English, and I think that it would do the devs well to get a good copywriter on board at some point to clean up the UI language for the English localisations.
Will I buy it? Probably not at this price, but I do respect what the developer is trying to do with Glyphs.