They are open year round and have the largest selection of costumes and accessories that I've seen. There are no dressing rooms, so you'll have to try everything on in the aisles. Each costume normally has a set specific for you to try on, but if you need a different size and they don't have a sample in that size then you're SOL.
The customer service left something to be desired. I went in yesterday trying to get an early start on a costume for my 5 year old. I found a wonderful costume online, but I'm often nervous about making online clothing purchases and not knowing if it'll be a proper fit, so I thought I'd see if they had it. I was immediately greeted by a man who asked if there was anything he could help me find. I told him about the costume and even showed him a picture. He said he'd check their computer and made a point of telling me that he'd find me in the store if I left the aisle I was in. I continued looking around, but after 20 mins...He didn't return.
I picked up a small mask that caught my child's eye and made my way to the register. The man was there along with SIX others, buy none of them said a single word to me. I awkwardly looked about me to ensure I was standing within the roped area that signals a customer is ready to purchase and still none of them asked to assist me. Finally, one of the pudgier guys who had walked back and forth behind the counter and made eye contact with me twice before...looked at me again and without saying anything just waved his hand to motion me forward as he rolled his eyes. I get to his register and he says "I'll help you." There was no mistaking his unpleasant attitude.
Most of the costumes here can be found elsewhere and much, much cheaper. I went to Party City before going here and Halloween Mart is selling the same exact Queen of Hearts costume for $40.00 more than Party City.
If it's not Halloween and you absolutely need a costume and cannot wait to have it shipped from online then, ok...you can go here as a last resort. It might be a better idea to try the costume on here and buy it online as Halloween Marr isover priced and the staff is rude.