What's the general consensus among the gaming community here on using non-official/non-GW models in your armies? There are some great looking models out there from other companies, and they would fit naturally in with 40k. However, since they are technically non-canon and not GW-issue, they...well, they're not GW. How do you feel about using these models?

Now, I'm not talking about integrating that other games rules along with their models. I'm just talking about using different models with GW rules.