In terms of the HDMI to Component cable. I think from reading up on them sometime back their only use is for PC graphics cards where maybe you have connected DVI-I to HDMI adaptor and then HDMI to component.
In terms of the HDMI to component boxes. I have bought a couple of them and they do work. But they can be a bit iffy in terms of HDCP. The boxes worked with my Roku/NowTV box. But did not work with my Amazon fire TV box due to an HDCP issue (it seems from what I remember reading the FireTV implement Hdcp slightly different than Roku boxes do as in the FireTV HDCP kicks in the moment you turn it on. Whereas NowTV/Roku only when you watch something which means the first doesn't work, whereas the second does)
There is picture quality loss, as in the TV screen is ever so slightly more blurry compared to the crispness of HDMI direct but its not that noticable unless you have it side by side.
They are worth the £10 I paid for it if you just want to have another box connected to your TV. Another thing you could buy the USB to 5.5mm DC power cables and connect it to a spare USB slot and have the device entirely powered by this. I say this as they usually come with american plugs and big adaptors for UK.
A final tip. If you really wanted to get the best out of your TV I would say possibly buy a cheap second hand games console for your movie watching. As these allow component out directly for DVD and Blu-ray watching. And even better if your TV has VGA input. You could buy an Xbox360 and use the Xbox VGA Cable which gives very nice picture and much better than component.