Long story short: I think the Princess Half is an AWESOME race and well, well worth it!! GO FOR IT!
I did the Princess Half this March 2010 and thought it was EXCELLENT. The weather is unpredictable anywhere you go... but you do have a good chance of good weather in March in Florida. It was indeed cold this year, but you really never know. Once I did the Disney Half in January and it was 80 degrees. Really, you just never know.
You do have to get up early, yes. But in my opinion that was not a big deal. Definitely not a dealbreaker. I've done several half and full marathons and you almost always have to get up very early for an early morning start. I myself would not use the wake-up time as a reason not to do such a fun event! I get up, run, get back to the hotel for a looong nap, and then I still have most of the afternoon and evening remaining to spend in the parks.
A Disney half marathon provides fantastic on-course entertainment (Disney characters cheering you on - some of them quite hilariously, I might add; parade floats along the route; DJs; cheering fans; live music; and more) as well as the great fun of running through the Disney parks and grounds! The race expo was a ton of fun too. Cinderella's coachmen were posing for photos at the door, the bibbidi bobbidi boutique ladies were "glittering" everyone for free... just really fun stuff.
The Princess Half Marathon medal is also great. It sparkles.... with little rhinestones. Oooh.

I wore it around the parks afterward (always a fun thing to do) and got many compliments and congratulations.
I definitely enjoyed taking a trip down to Florida for the race. I took a few extra days off and made a nice long weekend out of it. If it is truly an issue that you travel to Florida, maybe that's a dealbreaker for you. But if not, and if you can see it as a great vacation that starts off with a great run, then all the better!
I trained indoors through the winter in Michigan. Having a goal helped keep me in shape during the cold winter months this year.
Ok, enough of my gushing. Long story short: it's a great race!! I'll definitely do it again. I had an absolute blast every step of the way. I say go for it!
(and check out the WISH board for more info as FireDancer noted!)