Wow, that's a loaded question! I'm sure you'll get a 100 different opinions.

I had an Oberon and a clip on light with my K2. I thought I really like it at the time but I was always adjusting the light because of glare and there was all that added weight too.
When I got my K3, I wasn't sure what cover I wanted, so I tried to use the clip on light without a cover. It had worked fine that way on the K2 when I was wanting to lighten the load but the K3 was so much smaller that there really wasn't anywhere to securely hook it.
I finally ordered a lighted cover for the K3. It wasn't as bright as the clip on but it's perfect for reading in bed at night. It also doesn't create a glare on the screen and I was amazed at how light everything was. I've really fallen in love with the lighted cover.
When I upgrade to a K4, I'm sure I'll go with the lighted cover again. I love how light and compact everything is. Also, with the Kindle getting less expensive, I'm finding it more and more difficult to spent almost as much money on my cover as my ereader.