I've been cosplaying for 20 years and fursuiting for almost 10 of those years. In that time I've never worn a cooling vest. For various reasons *cough*age*cough* I've noticed a decline in my performance ability; I can't suit as long as I used to, require more frequent breaks and it takes longer to recover from an event or a con. I decided it was time to take the plunge and get a cooling vest!
Like headfans and camelbaks, cooling vests are entirely a personal preference. Depending on who I speak to, cooling vests are either essential or useless. I tended to fall in the later category, as I could often outlast any cooling pack's ability to maintain it's temperature and found cold packs very quickly became heating packs - not to mention the extra weight and bulk to pack for every con and the consideration as to how the pack may change the "look" or body shape of a costume.
During my research, EZ cooldown has constantly been praised as the best cooling technology for mascot performers, so it was my first choice for taking the plunge down the rabbit-hole of cooling vest technology.... FC was my first foray into suiting with a cooling aid... here's my completely unbiased review:

Purchasing:
If I need to start at the very beginning, I need to talk about purchasing the vest in the first place. EZCooldown's webpage (http://www.ezcooldown.com/) is easy to follow and explains all of the features and sizing, and the different types of cooling packs they make. For mascot performing, be sure to select the "Performers" vest; the other vests advertised are based off evaporation technology instead of cold pack inserts.
I'm from Canada, so I really expect anything I order online to take at LEAST 2 weeks to arrive. Once I placed my order, the vest was at my door in a mere 2 days!!! Wooo!
Vest also came with excellent typed up instructions for fit, storage, care and use of the vest and cooling packs.

Fit:
The vest is comfortable. It has large velcro straps to adjust the fit and size as needed. I can make it extra snug for a more form-fitting look, or loosen it a little to give me additional movement. Due to the bust, the front cooling packs are OK. With the vest on, the vest does flatten my chest to nothing. The packs in the back fit excellent in the small of my back with barely any volume (I do have a prominent bum, so it's possible my badunk-a-dunk are helping conceal the packs even better).

Movement:
As I mentioned in the fit section, I can increase or reduce movement depending on how snug make the vest in the first place. I wore the vest during my dance comp prelimns and found it did not hinder my ability to dance. I did choose NOT to wear it for the actual comp however since a lot of my dancing is torso work and I wanted to ensure my movements were exaggerated as much as possible.

Cooling ability:
There are a few different options for the cooling packs available with the vests. I got the Yellow (it cools less, but for a longer period of time) 6 cell (lighter and offer more movement) variety of inserts. I like to suit for a long time, so I wanted to get the most out of my packs. I also know from years of suiting that I can take the heat, so I didn't feel I needed the extra cold packs.
I can attest, these packs DO work. They were cool and comfortable to wear, sweating was reduced and I NEVER had to visit the headless lounge the entire con. They did not last the 3 to 4 hours they claimed. I often found they lost their cooling power after about 2 hours.

Cooling activation:
My largest concern getting a cooling vest is con convenience. Most commercial cooling packs require a freezer and a few hours to get properly cold. Having access to a freezer is pretty much impossible at a con, so I was only willing to make the plunge to cooling packs if they would be were convenient to do so. A few trips to the hotel ice machine and 40 minutes in the bathroom sink and my cooling packs were ice cold! This was VERY impressive! It gave me ample time to eat lunch and wait on my cooling packs. For intense suiting sessions purchasing a second set of packs could be very handy (one set could chill in the sink while I perform, then a quick switch-a-roo and I could continue suiting without waiting around).

Final verdict...?
After a con of wearing my EZ Cooldown vest I can honestly say I could take it or leave it. Haha, yup! As a hardcore suiter for many years I have come to know my body. I know what I can handle and when to take a break. If I forgot to bring my vest to a con, I would not suffer, but I may have to visit the headless lounge a few more times. I could see a cooldown vest being handy outside of a furry con, where headless lounges do not exist and taking a break would be much more challenging.
I don't like how the vest eliminated my bust, but that may be easily resolved by wearing just the back inserts. Also, like with all cooling vests, cooling packs DO become heating packs after a certain amount of time - this can increase discomfort if you cannot readily return to your room with any expediency.
I could also see a second set of cooling packs being handy to switch out instead of having to stop and cool the packs in your hotel room sink.


Pros:
Fast shipping, easy to use, cool quickly with limited hotel resources

Cons:
Some loss of body shape, additional weight, minor loss of movement in the torso


Hope this review was helpful!

Since purchasing my vest, EZ Cooldown have also added a neck cooling collar which I am very intrigued by... I may have to get one to compare with the vest >_<