I've had two iterations of this 90 gallon reef (moved it twice now), and I think my 2nd most favourite part about a pen style is the easy accessibility to all three sides for cleaning purposes. The downside is that you have to keep all three sides clean
You can actually view it on all 4 sides but I eventually gave up cleaning the West side of it where the overflow box sits. Most favourite feature is as you said, it allows you to cram stuff in on all four sides. I guess it is more of an island than a peninsula :-)
I am seriously thinking about starting a tank build thread for a potential new tank. Where the 90 gallon sits it is about 30 inches away from the wall so I could in theory put a new tank in its place that is a good 2 feet longer. What I'm concerned about is whether or not the floor would hold such a beast. I need to research that before I'd go forward...