In case you were unaware the EF D screen is the Precision Matte with Grid version. This overlays a grid pattern of 5 vertical lines and three horizontal. Theres a gap in the centre with a circle round the centre focus sensor (this circle isn't shown in the publicity diagrams).
The grid lines are EXTREMELY fine, unless you look for them they are almost unnoticeable (again, the publicity diagrams don't give this impression, they look thick on them). This makes them a bit hard to see against a very busy or dark background, but its a good compromise as they don't distract you from the viewfinder image. They are very useful for aligning buildings and horizons, and also for composition. Two of the verticals are on the thirds, so I find that I'm doing less cropping to get the picture right.
On a more light hearted note the centre circle makes you feel that you are looking through a telescopic sight, so it even satisfies your James Bond fantasies
Fitting the screen is very straightforward. The kit comes in a quality plastic case, and consists of the screen and a fitting tool with instructions. Simply remove the lens, place the camera on its back and use the hook on the tool to release the catch on the screen holder. The holder drops down revealing the screen. Using the tool (which has an arm which you push down to open the jaws) grasp the tab to the right on the screen, release the arm to grip the screen and remove. DONT TOUCH THE SCREEN as any skin oils will be extremely difficult to remove, if not impossible. Theres a place in the case to stand the old screen while you grasp the new one with the tool. Carefully place the screen in the screen holder (the reverse of removal). Then use the tool to close up the holder and refit you lens, job done. Using the tool grab the screen and put it in the holder where the new one was to keep it safe and dust free.
Theres a video guide made by the Canon Professional Network which shows the procedure, all the direct links to it were dead but you can find it on YouTube.
In conclusion, I'd recommend this EF-D screen. It has no drawbacks other than the lines being very thin, but many advantages. Theres a Custom Function setting on the camera to select so that metering is unaffected (unlike some of the third-party screens available). Its not cheap, Canons official price is £28 which for a bit of plastic is a bit scary, and one could argue that for the price we pay for the 40D or 50D it should come as standard. But, it does what it says on the tin and can only help your photography. I got mine from amazon and only paid £21 for it BTW.
. I think the only disadvantage is the price, but I think it is a 'Canon' thing, especially with regards to hoods for lens, but thats a different rant ha.