I, too, have a Red Sea S650. I do love the tank. I have also made many modifications to my tank including skimmer and lights. You can see my tank build link for more on those updates. For me, the modifications I made were for the following reasons.
1. Lights to LED for heat control (I have a JBJ 1/4 HP)
2. Skimmer - The one that came with it was very temperamental, hard to tune in and clean.
I did add Gyre pumps to my setup and stopped running the pumps on the back. Although, I do feel that with the return and those 4 pumps on the back, there was plenty of flow even for SPS.
I do agree, that for the money I could have built my own. However, having little experience with large tanks, the plumbing was a little intimidating to me to take a task like that. With plumbing on the RSMS650 already built in, it was a clear choice for me. I have learned a lot about how to set up a reef tank from this tank and for a first dive into a large tank I think it's worth it. The looks of the tank are classic and clean.
Enjoy the tank!