The question: What do you think about using variant army lists and Forge World rules in games of 40k?
Constantly Risking Obscurity
I'm not for using variant/Forge World lists against most opponents, and here's why.
Wargaming gives complete strangers a firm context in which to meet and play. When this context becomes unpredictable then the rock on which the game is played becomes sand. FW/variant lists are not widely known (at least in my gaming circles) and there is no basis by which to know whether your opponent is playing fairly or loosely with their esoteric ruleset. This does not apply to a close-knit gaming group or a specific challenger, of course. If you already have a trust relationship with your opponent, or if someone comes to you interested in fighting your unique army, then go ahead and play them.
It's like "house rules" in card games. Nobody joins a game with strangers expecting to use their own house rules. You use their house's rules, if there are any, or you play according to Hoyle.
The Midnight Shift
I personally don't have a problem with the variant lists/products from the Forge World line. A model is a model to me and if you want to try out a new rule set for fun, so be it. If, after testing, the new rules appear to be unfair/unbalanced, we'll talk it over and decide from there if we want to keep using them.
Please keep in mind however, that I play in my basement with a group of friends. We don't do tournaments at this time and there is no exchange of prizes or money. If I was in a situation like that (say at a FLGS during a tournament) then I would have an issue if someone was bringing a potentially "broken" or unbalanced list outside of the GW rule set.
The Vanus Temple
When the majority of players see a FW model on the board they think they've instantly lost, which of course is insane. The rules for the lovely pieces of resin are actually fairly balanced, usually based upon existing rules written by GW.
Sometimes I feel that the rules aren't powerful enough for the price you pay, and I think that's where people panic, thinking they do actually have awesome game breaking rules, when they actually don't.
The army lists produced by FW also aren't game breaking, true they're not as playtested as much as GW lists, but do you ever see a FW variant list that is made to win? No, as they're meant to be themed and for narrative play.