Hi sweck,
I will chime in here with a yes it is worth it. I bought it when it was pretty pricey and still thought it was worth it, As to the install I would save your all your files and get them to a safer place than just a floppy.
There is a provision in v5.1 to backward the EQ so you can "save" those sessions and if ever they need to be recalled you can recall them and they will sound the same. I am not completely sure how to do this but I think it is quite simple.
Then I would hassle the "they call it easy" installation of both v5.1 and Service Pack 3. I think it is a good idea to fresh install with a drive format this is why to save the files. You will not mess up the d8b by installing the upgrade. You could just install the OS but a safer bet is to do the whole thing fresh beginning to end.
Also you most likely have SP3 if in your set up now but there is no way that I know of to check this.
The best information available for installing the OS is here...
http://my.mackie.com/store2/p.asp?p=d8bos&t=2There is a set of instructions for whatever computer system you are using to download and save the images to floppy so you can load them into the d8b. What I do know is that what you save to the floppy is a image that is represented by a green icon, this is the correct file to get onto the floppy. Most modern computer systems automatically unzip files. A lot of it is common sense to be able to get this done. But if you do not see the green file you have missed a step.
Also on that page if you click "description"... at the bottom of that page, it says click here..., this gives you a tour of v5.1 another click away at the bottom of that page, that is entertaining and informative.
Hope this helps.
Michael