Since I've ordered practically every bag mentioned in this thread and have finally now tried them all out, I thought I'd offer a few more thoughts in case it helps someone. Unfortunately for my wallet though, since I recently decided the only way to know if a bag will work for Disney is by using it, I've now eliminated my ability to return many of them.
In any event, for now, after receving dozens of bags and after a lot of experimenting, I've decided to bring the REI Nikole day bag and the Ameribag Healthy back bag in a a size small, along with a cheap drawstring nylon Reebok bag that actually has two outside pockets that fit a water bottle (got it at Target) -- and I'll see which one works best for me while I'm actually in the parks. (Keep in mind I'll be keeping a big backpack in the stroller with my supplies, so this is just for my valuables and necessities while on line and on the rides).
Requirements for my bag:
#1: comfortable!!!
outside pocket for at least one water bottle
outside zipered comparment for small wallet
ability to wear cross body, and can be pushed to the back or be worn by my hip
lightweight and water resistant, wipeable
big enough for: two water bottles, cell phone, digital camera, mini camcorder, mini anti bacterial travel wipes, two snack bars, sunglasses (while inside on rides), tiny pouch of essentials with travel size sunscreen, extra pair of contacts, and various small items ... plus I'd like it to be able to accomodate a few extra items in case I find myself needing them (I don't want it to be too tight a fit).
I already owned the Overland Equipment Placer and Donner before reading this thread and have used both bags many times -- the Placer is too small for me to fit the above listed stuff, so while it's a very well organized little bag, it's just too small for my needs. And with no place to put a water bottle and a four year old who drinks a LOT of water on a normal day, it won't work.
As for the Donner, honestly, I've never really liked the bag. For me, it's feels too stiff, and with a water bottle in each side, there's not much room in the center of the bag -- plus I find it difficult to get the water bottles out of the side pockets for some reason. When packed, the strap is very uncomfortable for me and the bag feels a LOT heavier than I think it should for a bag that size. I know there are many fans of both the Donner and Placer, so this is just my opinion.
After really using the Baggallini messenger over the last two weeks, I am really disappointed to find that it felt like "too much bag" for me to wear in the hot sun -- it has a bulky feel to it, maybe because of the wide strap, I'm not sure. I found it very difficult to access the compartments while wearing it -- I was constantly taking it off in order to get anything out, which to me, defeats the whole purpose of wearing a sling bag versus a regular backpack.
On the other hand, the Ameribag Healthy back bag (Small) was actually very light feeling, didn't slip off my shoulder, was easy to swing around to get things out, it can be worn on either a shoulder (a plus for a right handed person since many of the bags have you getting things out of a sling bag with your left hand), and I found the outside pockets very useful. The XS would be a fine size to fit everything I list above, but I want to be able to use the bag for more than just Disney, and the Small accomdates the additional items I'd normally bring along, without really being that much bigger than the XS (I ordered both to compare).
The REI Nikole backpack does not have a great front outside pocket (it's shape allows things to spill out if you're not careful), but I love the option of being to wear it either sling or backpack style, plus it's very comfy to wear and has a great bottle pocket and a back zippered compartment for safety. I'm bringing it in case I end up not liking wearing the Ameribag sling all day since I've never worn a sling bag style before for any length of time.
I also tried a waist pack, the Jansport Coltrane, that I thought I'd love, but unfortunately, with what I want to carry, it won't work. When packed, I couldn't sit down comfortably, and the bag tipped forward while walking. While I LOVE being hands free, I know I will not be comfortable wearing it while sitting. I'd love to find a waist pack big enough to accomodate my stuff that doesn't bother me while sitting, but I don't think it's possible with the amount of stuff I need. I do wear a great Kipling waist pack while at museums and traveling, but it doesn't have a water bottle pocket (and won't fit one inside). I have two other waist packs with water bottle pockets, but they don't have enough room for the things I need to carry with me.
I'm curious about the Travelon expandable messenger, but I had gotten a Travelon sling backpack in the microfiber and didn't like the material.
Anyway, HTH someone! I keep reading hoping to find new bags to try, but it seems we're all trying the same ones!
****Edited to add: After reading through a few more posts, I just ordered the following waist packs so I can comment once I recieve them. I'd love for one of them to work because I don't want to feel weighed down by my bag:
Mountainsmith Kinetic II:
http://www.ebags.com/mountainsmith/kinetic_ii/product_detail/index.cfm?modelid=70072
Jansport Revelation:
http://www.shoebuy.com/jansport-revelation/152230/352472
Great pockets to seperate items, useful attatched wallet inside for xtra $, durable, really pretty, plenty of room. 
- do you suffer from this same problem??
Don't get me wrong, I love the accessories - I don't know if this is the case with all of the prints, or just the one I purchased (capri blue), but I would really like the bags to have a different print on the interior, so I am able to find the accessories a little easier.
He really is wonderful. You'll have to let me know how you end up liking it.
