I've read that beer syrups made from IPAs have an astringently bitter aftertaste, but after a few trials, I haven't found this to be true. This may not work for all IPAs, but I saw that this recipe works best for stouts, ales, and porters. Most recipes that I read said to use equal parts of sugar and reduced beer by volume, but I didn't want my syrup to be sickeningly sweet, so I used half the amount of sugar by volume. And yes, I actually weighed it.
1 - 12 oz. bottle of IPA
1 Tablespoon of sugar
Reduce the beer with one tablespoon of sugar over medium-low heat, keeping it at a gentle simmer and stirring frequently. This should take about 20 minutes, depending on how many bottles of beer you use. Once reduced by about half, weigh the syrup and slowly add sugar to taste. For example, the first bottle I reduced ended up weighing 6.4 ounces, so I added in 3.2 ounces of sugar. Whisk until sugar is completely dissolved- this works best if the syrup is still hot.