On the fence for the S&W version. The last couple of years of releases have been hit or miss for me from the Frogs. I wasn't overly impressed with the Rappan Athuk Expansion, Sword of Air, and Tome of Horror IV. Although Cyclopean Deeps was quite good.
As the author of Cyclopean Deeps and not of the others, maybe I've got some credibility on the evaluation, then.
I'm with FGG, so take it with whatever sized grain of salt is appropriate.
I'll also mention a bit about converting to 5e. I'm the one doing most of the conversion to S&W from PFRPG, so I'm very familiar with the book.
Conversion
First, as to conversion into 5e, it's not specifically designed for it, so the Pathfinder version is about as convertible as Pathfinder conversions tend to be, and the Swords & Wizardry version is about as easy to convert as a normal S&W adventure. If you don't like converting adventures into 5e, it's an awfully expensive resource book just for cool ideas. If you're really rich, buy it; if not, it's probably too pricey for that particular use. For conversion into 5e, my own preference is to convert from Swords & Wizardry, since the monsters are at roughly the same level of complexity, and although there aren't skill checks, all the traps and challenges still either have a numerical component (if I thought it was needed during S&W conversion) or are described with enough detail to make it fairly easy to assign a difficulty.
As to the adventures themselves:
I'll quote something I posted on the Paizo board when someone asked a similar question. This is how I see the overall feel of the series, although it's obviously "rah rah" language:
It's kind of like this:
Take the Elder Eddas.
Now put something very horrible that happened in the past, creeping back into the world of the Elder Eddas.
Sprinkle with just a hint of Robert E. Howard.
Add a possibly-contested succession to the throne to a long-standing feud ... juuuuuust until you're at the very brink of a three-sided war.
Add monsters.
I think part of the concern about the book (other than the pure price point) is that people are assuming it's a reproduction of a historical viking world with a leavening of textbook monsters. It's much more than that; all of the adventures are caught up in Norse mythology, but with a Conan-esque, sword and sorcery underpinning. The only one I didn't really like was unfortunately the first one in the series. I think I fixed it for S&W, and I assume it works well as written with PFRPG, though I can't speak to it one way or the other. I just felt like that adventure didn't work at a beer-and-pretzels level, which IMO is a required foundation for a more old-school feel, even if your players build from there into something richer. The second half of it is quite exciting; it was the first half that didn't entirely jibe for me.
All the other adventures have a strong old-school structure: a location-based desperate situation, then a nice sandbox, then a follow-up assault to kick the enemy's ass on his home turf after the sandbox, etc, on toward heroism. None of the adventures feel like "Okay, we're vikings, let's do viking stuff according to history." I haven't read any of the other Viking modules that people listed, but I can definitely say that there's a creative avenue taken on this one that distinguishes it from just "I know lots about Vikings."
That said, of course, Cyclopean Deeps is the absolute masterpiece of masterpieces, and the author is utterly brilliant, so look into that, too.
