I think The Blade Itself pays off. In an attempt to not spoil things, the first book is mainly "getting the gang together." The second is "the gang goes into action," and the third is "the giant climax."
Of the three standalone novels, "Best Served Cold" is probably the best and an entertaining read. However, without having read the original trilogy, you'll likely be a bit thrown off by many of the names and events referenced and have a couple of plot points of the trilogy spoiled for you.
I am currently very high on the First Law series, but I will admit I first read The Blade Itself almost six years ago and did not like it. But I think if you take book one as table setting and let yourself get into book 2, the merry crew of Glokta, Logen, Byaz, Jezal, Ferro, Collem, and all the rest will give you an unforgettable story.