It's like eBay in that it's a hub for individual companies and sellers. So it's only as trustworthy as the individual seller. Check feedback and all the sellers I have used are eager to communicate both before and after the sale. I've placed a lot of orders on Aliexpress with probably 85% satisfaction, mostly clothes, shoes and jewelry. I always request more detailed photos and "real" photos from the seller before placing an order.
If something is wrong with your order, you can open a case directly with Aliexpress because they own Alipay, their payment processor. I have read complaints that they favor the seller, but I have gotten a refund or partial refund on the probably ten cases I've opened. I have only had one item that wasn't received at all. Most items were the expected quality. A few surpassed my expectations. A handful were far less quality than expected based on the photos, descriptions and feedback; those are the items I filed cases for.
The thing is, sellers will often agree to a return if you ship the item back, but the cost of shipping to China usually exceeds the original cost of the item. You can request a refund without returning an item but you really have to push sellers to do that. You can usually get a partial refund and keep the item so as not to pay the shipping back.
But yes, the site itself is legit. I have had no fraudulent charges. it is a subsidiary of Alibaba.