I'm runing this campaign with 3 party (now, they are all in the third module, backland). The campaign is good at first glance, BUT... it has a few problems
*It is written for AD&D, and the 3e adaptation is not complete, nor perfect : you will have more work than you can think.
Plus, the authors decided to not use anything in the FRCS 3e : thus if you want to keep it compatible, you will have more work (return of Bane, Manshoon wars, Red Wizards enclaves, Shades in Anauroch, drow in cormanthor...). The "Immaskari magic is not from the weave" thing is especially annoying, since in 3e the weave is everything, with the exception of shadow weave. I think I will rule that the sigyl are shadow weave enchantment : IIRC, shadow weave magic was known by the Talfir, who learned it through Unther, and so through the Immaskari. I know the shadow weave is a recent thing, but what is recent for a godess who was here since the begining of the world ?
*overuse of powerful NPC : the PCs are often mere spectators and have nothing to do but wait until the DM end his "power trip". This is especially annoying in the first part (Sylverymoon) : Agatha and Xred, Manshoon, the Harpell, the gnome clerics and the waterlingorn, the cultists and the draconians... They are all level 10+ (or 18+...) while the PC are 1st or 2nd level.
So I have to make some changes (or will make if I ever replay this campaign) :
-Agatha and Xred : to pass the magical barrier used by Xred, Agatha need the help of the PCs, who will place a magic trinket in the center of the orc's camp, allowing her to teleport inside.
-The Harpell and the Barbarians : I will remove, or change radically this part. While it is fun to read, it's really frustrating for the PC. Such show of powers are somewhat useless for the story.
-The cultist attacked by the draconians : I made the PC arrive AFTER the fight, so they have to figure what happened, rather than to be slaughtered by level 12+ characters (guess what is the result of a 12 D6 lightning bolt upon 2nd level PC ?). If you want a fight, you can make one of the cultist survive (trough a feign death spell), and reanimating his buddy to bring them elsewhere : the PC will see an evil cleric animating corpses and will probably attack the (already wounded and weakened) necromancer if they are good.
-The gnomes and the Waterlingorn : make the gnomes and the beast lower level, so the PC can help to fight, and have a real role in this part.
-In part three (the backland) I simply removed the half-elven ranger and the two elves : that's three NPC less to manage, and you just have to put more reasonable odd against the PC.
*Overuse of some type of events : "you are surrounded by 300 orcs/gnolls/orcs, they want you to speak with their leader, you have no hope of escape nor victory against such a number" : it happens once in the first part, once in the third, and once again in the fourth. I will probably remove the orc encounter in Cormanthor, and the PC will be bringed in Dekanter by a single (powerfull) gnoll, who will negociate with them ("wanna gold ? wanna slaves ? my chef will give you if you come" It has worked with the first party, because they wanted to free the elven slave the gnoll promised them. They could not guess the elven slaves were dark elves...)
*It's a long journey : travelling the wilderness for weeks can be very boring, when all the encounters are hostile orcs/goblins...
*It's an uninterrupted journey : if the player want make a pause (by example for crafting magic items, or simply scribing a scroll or two) they can't.
*It's a journey : travelling through the norther half of faerun is great, but my next campaign will revolve around a single spot and it's surrounding : this let the players learn where they are, and use this knowledge.
That's all I can think now. The Northern Journey is really a great campaign, especially if you play with Ad&D2 or are willing to add a lot of work. I just wait for the release of the last parts...