Online Unlocking Service Recommendation

I have always bought my phones unlocked, so I never really have to deal with finding an unlocking service. However, just for my knowledge for US phones, who do you guys have used? Any experience? Good/bad? Random shady Ebay ones? Seems like all the websites offering unlocking service look shady with the type of graphics they use, animated GIFs, dozens of logos to show you how "secure" they are, etc.