1. Has anyone used the extension tube offered on Amazon made by Kenko? -
I have no experience with these extension tubes but, have heard good things about them. However, I was able to get a 90mm f/2.8 Tamron AF SP macro lens on eBay a couple of years ago for less than $125 with shipping. This might be an alternative to extension tubes if you can find one of these great lenses used (note: this is not the Di model but, has outstanding image quality on my 1.6x cameras). Definitely avoid the el-cheapo extension sets sold on eBay. They are trashy.
2. Are pop-up screen covers worth the money? And if so, what brand is recommended?
I bought a Delkin pop up screen cover for my XT and didn't really like it. I don't chimp, but will often check the screen to make sure that my camera is working properly. In order to check it with the Delkin, I have to pop open the screen and then close it before I shoot.
3. Is the 90 degree angled viewfinder worth it? Seems useful... but haven't ever used one..
I use my 90 degree finder fairly often when I am shooting with a short, but very light weight, tripod or when I am doing low angle macro work. This saves my back from straining. I purchased a Chinese knock-off on eBay and it works just fine. However, when I bought mine, I didn't realize that it did not have a 2x magnification capability. I would definitely choose a right angle finder with that capability. Another use for a right angle finder is when shooting people with a wide angle lens. I am a tall guy and if I shoot people or groups of people using a wide angle lens (sometimes there is no choice because I cannot back up or arrange the group any tighter) with the camera at my eye level, the natural distortion will make the subjects heads and torsoes seem much larger than their legs and lower torso. The right angle finder allows me to shoot from about their chest level which decreases the natural distortion somewhat.