I took it in 2 tests, and didnt do well on the ICND2...wish I could have done one test, to be honest.
I took the first 2 cisco classes at a community college at a regular pace, and decided Id take the first test since...I could and knew Id be able to pass it (I also required at least a CCENT or Net+ for some courses down the road and wanted to be sure I met that)
I had to take the last 2 courses independently, in one semester...I pretty much rushed through the material and then took a break due to another semester of school and was rusty when I took the ICND2 (even with a fair bit of studying)
If youre experienced and have done some regular studying, or if you took classes and did them all in a row without rushing...screw it, take one test, save a few bucks, get it over with. 2 tests is more questions and more money.
I really, really wish I had been able to do all the courses in a reasonable time frame and taken one test right away. Id probably have a CCNA by now instead of just the (essentially worthless) CCENT