Generally, from a gaming point of view, using Forge World bitz to improve the look of our models has no real advantage. Indeed, in some cases it's a disadvantage? Take the Forge World Eldar Avatar for instance! Although it is an awesome model, it's at least double the size of the GW model, and twice as difficult to get 50% behind cover!
Of course there can be some advantages as well, my Forge World Eldar tanks are slightly longer than the GW ones, making it a bit easier to contest two objectives! But this is not the reason I went down the Forge World route! So why did I? Resin can be a pain to use, and it costs a lot more money?
Well, my first army was Necrons, and with only a Monolith as a tank with no FW upgrades, I have to admit I got a bit jealous seeing my buddies upgrading their tanks and dreadnoughts with FW bitz. Necrons of course only had the Pylon as a Forge World model, and at that time I only ever planned on having one army (like you do when you first start playing) so I treated myself to an expensive Necron toy.
It wasn't long after getting my Pylon, that I got the new army bug and it was time for me to get a second army. This time I wanted something different from the stand and shoot Necron's. I looked at Ork Speed Freaks and Eldar Saim Hann, but in the end I settled for the Eye of Terror's Elite, Space Wolves 13th Company army. I was playing alot against a hard hitting Emperors Children army at the time and I wanted some CC revenge. Trouble was, no tanks!
My third army was a Nid army that I got very cheap from a mate of mine. It was basically a massive box of Tyranid bitz, some made, some not, but definitely no tanks!
So that was it, my forth army had to have tanks, and the IDIC Eldar Craftworld was found. After waiting so long, I knew that my Eldar tanks were going to be all Forge World. It just had to be done, and that is why Forge World...
So what do you guy's think of FW models? have you found any advantages or disadvantages to certain models, and with the influx of models like the Mawclaw/Trigon from GW, do we really need to buy FW anymore?