As someone who owns both a Drobo and an unRAID box, I strongly suggest going with the unRAID solution.
Drobo sounds like the absolute perfect product for anyone not wanting to manage data and just have things work, but that is not the way it ends up in the end. I haven't checked the Drobo forums in a while, but I believe they are still locked and require a serial number to enter and browse. This was done for a reason and don't let anyone fool you into thinking otherwise. There were tons of complaints about speed and data loss - my two main complaints as well. Any company that won't open up their support forums for pre-purchase support needs to examine their product and practices. You can check my posts on that forum if you have access, they are under the same username as here.
Regarding Drobo support, the typical response time from the company was anywhere from 3 days to never. This is as of 8 months ago, so things may have changed, but it was near impossible to get someone to actually support the product. Unlike with unRAID, there was no way for anyone in the forums to really help you fix a problem or get back your data in case you did something wrong. The RAID format is proprietary and the user community has absolutely no control over data recovery. There are no tools in case anything goes wrong. Your only option is to contact support and wait for a response. Oh and I believe they started charging for support past a certain timeframe if you do not pay your yearly maintenance fees - Something unRAID does not do!
I have purchased 2 different Drobo's, the USB only version (returned after 3 weeks), and the firewire/usb version (still have it). I really wanted to love Drobo. As someone who has worked with literally hundreds of different RAID configurations, I had hopes that things would be easier for the end-user, but I now feel their technology is not destined for the enterprise. One last note on Drobo, if you are using it in a Windows environment, you will never see Firewire 800 speeds. The Firewire driver for Windows XP/Vista only supports Firewire 400 - I'm not sure about Windows 7. I didn't believe this either until I read about it in their support forums and searched the internet.
The write speeds of Drobo (firewire version) are comparable to unRAID as long as you are writing only one file at a time. Once you start writing multiple streams to the Drobo, transfer rates to downhill substantially - this is not the case with unRAID from my experiences. Read speeds from Drobo are the same as writing, which is where unRAID really kicks in. Read speeds for unRAID are 3x-4x what the write speeds are, so unRAID is the clear winner here.
Regarding unRAID, yes it does require a bit of knowledge to get set up, but not any more than installing Windows or building a small computer. unRAID is simple enough that after it is set up, it just works. If you lack technical know-how, one of your computer friends should be able to get you started and if you use a case with hot swap drive bays, you should be able to add or replace a drive without any help from him/her.
The biggest thing unRAID has going for it is the community support. There are several members on this board who answer questions in minutes and take the time to walk you through any issues you may have. They have helped me a few times and I can't say enough good things about the folks here.
All that said, if you want to buy a Drobo, PM me and maybe we can work something out. Mine is being sold to make room for a second unRAID box