The original 5D is a wonderful camera. If it seems less than stellar, it's only in comparison to its successors.
It's important to understand what its successors offer that it doesn't. And, if nothing that's been added is of any consequence to you, then there's nothing to be gained from the new bodies.
First is resolution. But, if your printer sits on your desktop -- that is, unless it takes ink by the gallon and paper by the yard -- then you'll never be able to tell the difference in a print. (Of course, if you crop heavily, you might need more resolution.)
Next is high ISO noise performance...but, again, this is largely determined by print size. If all you're doing is 12" x 16" and smaller prints, you can really push the ISO even on the original 5D and you'll be just fine. It's only at the larger print sizes that the high ISO noise starts to fall apart.
Then there's autofocus. It's barely adequate on the original 5D and not much better on the 5DII. The 5DIII has the second-best autofocus of any camera ever made, only barely behind the 1Dx. If autofocus at all matters to you, that's plenty of a determinant right there.
Close on the heels of autofocus is frame rate and buffer size. It sucks on the 5D, it isn't too terrible on the 5DII, and it's plenty for all but the SI crowd on the 5DIII. Indeed, put the 5DIII's autofocus and shooting speed together with its high ISO performance and image quality and the rest, and it's probably Canon's #3 sports camera of all time, second only to the 1Dx and the 1DIV.
And last is ergonomics, especially including things like the LCD screen and buttons and all that stuff. There's no question but that there's been a very steady improvement, significant bordering on dramatic...but it's not like there's anything horridly worng with the 5D. It'll get the job done just fine, so long as you know its quirks and how to live with them.
So, yes. The original 5D isn't just a good option; it's a very good option. That the 5DII and 5DIII are excellent and unbelievably fantastic options doesn't make the original 5D bad; it's still every bit as good as the day it was released, and it was revolutionary and very, very good back then.