I haven't personally used Viking cards in my FP6900, but I don't recall having seen anything negative about them. I have noticed that Amazon has a really attractive price on them right now, after you figure in the discount and rebate.
I don't think it should matter too much which Smartmedia card you use. I've been told there are only 2 or 3 actual manufacturers of Smartmedia in the world, and everything else is post-manufacture relabeling.
If you become concerned about the Viking cards for any reason, you might want to check out the 128mb Mr. Flash (terrible name) Smartmedia cards at newegg.com as an alternative. They come with a great guarantee and only cost $52 + $5 shipping. I have 2 of them currently and have never had a problem with them. If you do a search in the Fuji forum, you will find that many other posters here have used them too.
Please report back to the forum if you use the Viking cards. I'd like to know how they work out. I could use 2 or 3 more myself.