Another advantage of enclosed trailers is not having a bug covered bike by the time you get to your destination, but the price is a big turn off and not a lot of us have the cash to invest in those beauties...especially someone like me that only hits one track weekend a year...
My problem wasn't the initial investment, it was space. I don't have room to store a regular trailer, and I'm not willing to pay for a storage spot for one either.
I'm also picky about trailers. I grew up racing small sailboats, and my parents and I towed boats all over the place. Very few things will screw up your day faster than a trailer problem. Because of that, I'm fairly picky about things like wheel size, bearings, and the overall construction of the trailer.
As for the Harbor Freight trailers...I know people do it and haven't had problems, but personally, I wouldn't even put my mother-in-law on a Harbor Freight trailer, much less my motorcycle. Those things are the epitome of cheaply-constructed Chinese crap.
Because of my space requirements, I went with the Kendon Dual-rail. It is a quality unit that tows well (we took it 6000 miles last summer) and doesn't even mind having a heavy bike on one side and nothing on the other side. One person can stand it up on its tail and roll it around to store it. My only gripe is the stupid bling diamond-plate on the deck reflects the sun into your eyes if you're loading bikes on a bright day with the sun high overhead.
If I had the space to store a trailer, it would have been a toss-up between a covered trailer and a normal flatbed utility trailer. The covered trailers are nice because of the security, and because you can leave your track bike and all of your track crap (gear, tools, spares, etc) in the trailer all the time. "Getting ready to go to the track" means just hooking up the trailer.
The downside is a covered trailer is more limiting with what you can put on/in it, so you might not be able to get as much use out of it as a flat-bed. That may or may not matter to you, of course.
If you're just going to do a few trackdays a year, consider renting. The little Uhaul motorcycle trailer is just fine, and you'd have to do an awful lot of trackdays before the rental fee exceeded the purchase price of buying your own trailer.
As for a tow vehicle...unless you're doing long distances and/or frequent towing, don't worry about it. I used to tow a boat trailer that weighed about the same as the 600rr and my tow vehicle was a Saturn SC2. It did just fine and was still going strong with absolutely zero problems when I sold it at 108K miles. People who buy big SUV's and pickup trucks to occasionally tow dinky little motorcycle trailers a couple hundred miles are idiots.
