I studied spring of my junior year, which was the traditional time period at my school. I also went to Japan, and they have a very different semester schedule - i.e. spring semester starts in March-early April, so you either go for a whole year or the spring semester.
I personally wouldn't recommend going spring of your senior year simply because it will really break up your undergrad experience. While they always talk about culture shock upon arrival to the host country, you DO experience an adjustment when you return to the US (some places, like Australia, aren't too bad, but Asia, for example, is a drastically different environment). The first few weeks you might feel out of place and have difficulty relating to your peers who stayed home - and you really don't want this to be during your senior week and graduation. I felt that I really bonded with my classmates in my senior year, especially the spring semester, and I would strongly suggest going before your senior year, if possible.
I went with CIEE-Tokyo. The program was sponsored by my home institution, so all the grades and credits transferred (though I had to e-mail my intended registration there to have it approved by my advisor, since I studied abroad for my major, but it was OK'd swiftly). The grading system was slightly different - no +/-, but it came out ok, because I had 2 solid As, 1 A-, and 1 B+, so the A- became an A and the B+ became a B, which in the end is the same as if I had actually gotten an A- and a B+, GPA-wise.
And I always talk about this as a precaution. My *personal* experience was very negative due to being placed with a host family from hell. It took FOREVER for me to move out, and during the process I was dragged through dirt both by my host family and my program. For example, I was blamed by my program director for being rude, antisocial, unappreciative, etc, while my host family was painted as angels. I even asked if someone had complained about them before, and I was told "NO, YOU are the first one who seems to be incapable of getting along with them!" To give you an idea of what it was like - my host mother decided to make up a sheet titled "Things I don't like about Jochi1543" and kept adding items to it and reading it to me daily.
Anyway, then I managed to get in touch with 2 people who had lived with these people before, and BOTH told me they had a terrible experience and strongly suggested to the program that these people not host anyone anymore. So much for the "you're the first one to complain." I really made a mistake by waiting too long and blaming myself for everything when in reality, I played a much smaller role in creating the conflict. In fact, I underwent so much stress that I developed major depression when I got back to the US and nearly dropped out of college - but luckily, I decided to go see a shrink and was diagnosed and prescribed antidepressants, so I was able to get back on track and finish the semester (and my senior year) with a 3.75, albeit with 1 dropped course both semesters - I had to limit my workload.
Bottom line: you CAN run into serious trouble when you study abroad, but you CAN take control over it if you recognize the problem soon enough and change things that are making you miserable.
