I have a campaign that has been running for almost 18 months now. It is getting to be more fun as it progresses. My character group started out weak, idealistic and willing to do anything to help the larger society. Over the course of the campaign they have grown wealthy, powerful, cynical and mean. Most are heavily scarred and many a Lifegiving spell has been performed as almost every one of them has run out of fate points at one point or another. Despite these changes, they still manage to mainly be of positive service to the society.
I converted my group from D&D when the group began complaining about how limiting the game was in terms of character development. I intended to take them to Rolemaster or Stormbringer, but found HARP and decided to give it a try. The first session was a disaster, but once I got the hang of what needed to take place to run combats, I was sold. The group took a little longer and the main selling points for them are the flexibility of character development, the one hit kills and the magic system.
I agree the math can be quite a chore, but you can use spreadsheets or other tools to do the math for you. You could also have each player figure out final mods etc. while other players are having their turn.
Macros on roll20 are pretty useful for doing the math, but the setup is a pain. There is also the fact that there is no Harp character sheet for the tool. I currently am working on converting one of the HTML character sheets that I downloaded from the vault to take input. I am not going to begin working on the macros until the start of next year. I am currently not a subscription user and will not be until then.