I'm using inkscape to design a UI for industrial applications, and am having a hard time with simple line drawings. What I am trying to do should be very easy, but I am obviously missing something. Let;s start here:

Let's say I have created two straight lines in the shape of the letter "L". Instead of a sharp corner, I want to round it (symetrically, if possible). In the cad world, this would be a "fillet". I assume that I have to use one of the node tools, but no matter what I do I either get only a chamfer or a very irregular curve.

Can someone help out a novice? Thanks!

There are more ways to do this

One way is to draw a rectangle with rounded corner , adjust the rounding handles to get curve radius as you like (if you hold Ctrl while dragging rounding handles you'll get it symmetrical), then just cut the rectangle with another path to get the L shape.

Another way is to use Bezier tool to draw a 2 node path: Click to start, move mouse diagonally and click again, then right-click to finish path. This will give you a backward slash.
Now use Node tool to tweak the nodes, select the top node and hold Shift while clicking on it and dragging away, this will give you a handle. Do the same on the other node to get the handle. Now you can play with the handles to make the L with the rounded corner. Hold Ctrl down while playing with the handles so you can get them horizontal and vertical. You'll probably have to extend the handles beyond the bounding box to get what you want.

Although there are more ways to approximate this, I don't agree that this can be done in an exact way. It would be very nice to have this feature. Using a rectangle to start with is often not an option, and 'playing' with the handles will not give an exact radius, or even an exact circular piece. Combining a circle with a straight line is also not possible, except by making the circle a path, then cutting it and then connecting the part of the circle with the lines, but I find this is next to impossible to do exact with lines under oblique angles.
Definitely on my wish list, together with connecting a line tangential to a existing circle or path...

See the blueprint "A new tool to fillet or chamfer the cusps of paths". This is still at the planning stage, so if you have any suggestions about how this should work, now is a good time to post them on that page, or on the associated wiki page (or on one of the attached bug reports, if appropriate).

This same question was asked just a couple of days ago. You may find the information in that thread useful - viewtopic.php?f=5&t=880