I have been using Cost Plus, and it's been a totally positive experience. I became Medicare eligible this year, and the prescription plans all seem to pretty much suck. A medication that cost about $50 for a 3-month supply under my former employer-sponsored plan was going to go up to several hundred dollars under my Medicare prescription pan. With Cost Plus, it is costing me $95 for a 3-month supply.
I had to create an online account, download and complete a form, send it to my MD's office, and ask them to send the form and prescription to Cost Plus. Then I log into the Cost Plus website and purchase the medication, which is mailed to me. I highly recommend Cost Plus, if your medication(s) is available from them. That's the one downside; they don't carry everything.