My group and I love WHFRP2- its replaced D&D for our fantasy gaming needs. I've been running a campaign using WHFRP2 for almost 2 years now. The system is simple and intuitive (meaning almost no referring to rulebooks during the game), provides plenty of options for character development, and is a breeze to prep for. My group were pretty die-hard D&Ders, but after trying WHFRP2, we've been playing it and haven't looked back. Some strengths of the system:
1. Character creation is FAST and easy, while still being detailed. Characters have very different skills sets and abilities, and they start more competent than a beginning D&D character.
2. Characters don't "level" as quick, and don't become demigods after gaining a lot of experience. A heavily armored warrior facing 5 or 6 weak adversaries is still in a LOT of danger of being hit, and if one or two of them get lucky shots (usually involving Ulric's Fury- expoding 10s on damage dice), that character can go down.
3. Combat is lethal, brutal, and short. Wading into a horde of enemies is suicide, as is charging a battery of archers across open ground. WHFRP2 encourages smart fighting- teaming up, using cover, ambushes, etc- "heroic" characters are often dead characters.
4. Morality in WHFRP2 isn't black and white- its various shades of grey. Even the most well-intentioned character has his flaws, sins, and vices. Even Chaos, the ultimate adversary, can be viewed in positive terms or having positive results in some situations.
5. Insanity! Character can and will gain mental derangements as a consequence of their adventures, experiencing horrific things, and suffering critical hits. The various insanities are rarely crippling to the character, and are darkly humorous- this is one aspect my group has particularly enjoyed.
6. The magic system is very well done and balanced. Not balanced so much in a spell points kind of way, but in that it requires the wizard to assess whether magic is a good idea or not at a point in time. Magic is NOT a good solution to all problems- overusing magic can result in severe spell mishaps (called Tzeentch's Curse), mutation, and madness. In fact, the wizard in my group has developed a phobia of loud noises, and only uses spells that are quiet or cause minimal noise (meaning he almost never uses lightning anymore!).
7. The WHFRP world is awesome in its depth of detail. In addition, its familiar to most players since the cultures are loosely based on cultures of Medieval/Renniasance Europe. The tone of the world appeals a lot to us too- its kinda like CoC, Black Adder mixed with Terry Pratchett but with darker humor, and a good mystery in its tone (meaning WHFRP2 lends itself very well to investigative types of adventures).
8. The system is intuitive and simple. Its a percentile based system, and the GM is encouraged to keep things flowing and not bog things down in charts and rule-checking. When I started running WHFRP2, I checked the books maybe 4 or 5 times a session. Now, if I check something once, its unusual. That lets me focus on the game and the story, and the players love it. I've also noticed my players don't try to "build" out their characters from creation like they did in D&D. Instead they tend to grow more by their experiences, and feel more like real people.
9. Monsters and beasts ARE scary- charging to attack them directly will often result in a messy death or gruesome injuries. Even a bear or other large carnivore can be terrifying. A mob of orcs on the rampage is a scary thing too- they are bigger, stronger, and tougher than humans, and toe to toe, nothing other than a dwarf or chaos warrior can usually match their tenacity and power.
10. Its very well supported too. Black Industries releases one book every month or two, and most of the books have been very good IMO. Must haves include the main rulebook, Sigmar's Heirs (Empire sourcebook), the Bestiary, Tome of Corruption (book of chaos), Realms of Sorcery (magic), and Children of the Horned Rat (Skaven). Even the weakest books they have released- Karak Azgal and the Armory- still have a ton of useful information, and we find WHFRP2 books give us more bang for our buck than any WotC book has. Also, the Black Industries website has over 40 FREE full-length adventures that are very well done for download, written by the developers and fans, with one or two more added per month!
Possible weaknesses of WHFRP2:
Well, for us there aren't any really. But if you like a heroic game or lots of dungeon crawling or combat, WHFRP2 isn't the game for you. Its dark, gritty, and deadly. Character professions are roughly balanced in their advances and abilities, but NOT in their combat prowess- and thats a refreshing change to me. Also, if you're looking for a CR system in WHFRP2, there isn't really one- its expected if characters run into something they can't handle, the RUN! Finally, there aren't many magic items in the game, and getting rich off adventuring isn't likely- more likely you go insane or die. While these are all strengths for me and my group, if you want the same themes and play out of WHFRP2 that you do out of D&D, you will be disappointed.