Sometimes they are amazing, but lately I haven't been impressed. I ordered 3 t-shirts in what appeared to be a single store, but they came from different places and took 2 weeks to get all 3. Nothing custom, just stock logo shirts.
But point is they are generally a good resource for a lot of things. The wife and I have held off on the "prime" account because we'd feel tempted to buy too much stuff.
One thing to watch is if the item is stocked by Amazon. If it is, delivery will be quick. If not, the order is forwarded to the company supplying it and will be dependent upon their service standard.
I use Amazon Prime also. On covered items, which is a good many, it is free 2nd Day Shipping.
I also got their Credit Card. It gives "credits" of 3% for purchases from Amazon, 2% for any gasoline purchase, and 1% for other purchases.
Started buying gas with the credit card...had used cash prior to that. It is something you have to buy and I'll take the 2% rebate.
Also use it for all my Amazon purchases.
My regular credit card has the equivalent of the 1% rebate, so I use it elsewhere.