Reactors have a very high up front cost but pay for themselves in 3-4 years depending on how much 2 part you end up needing. Some people dose 100mL a day and when you're doing that you should seriously consider a reactor.
If you watch this, Dave at VividAquariums explains why his store only uses Dosing Pumps now.
http://www.youtube.com/watch?v=U9yFdBnY-xA
I personally dose for 3 reasons:
1. I can split dosing into as many or as little dosing sessions as I like via my apex controller.
2. I can easily increase or decrease dosing amounts by increasing or decreasing frequency or time I dose.
3. Calcium reactors can take a lot of "fiddling with." This was the big "NO" for me. I have never owned a calcium reactor but I have heard stories from local reefers about how much they have to play with their reactors. I believe in the acronym "KISS" (keep it simple stupid).
Watch the video, you'll see what I'm talking about.