Careful with Drobo. When they are working, they are great. But just know going in they use a proprietary disk structure, so if you have issues and need an emergency recovery if the array fails you can pretty much forget about it. I was burned once, and I am more careful now.
I have been using ReadyNAS since their Infrant days, they are not too bad.
I am always looking to build a more perfect beast, I am debating giving Synology a go. I hear great things about them...