I don't know of pencils with undo, but here's one with "replay." (If you point at [X] in your notes, it will play back the recording of the lecture at the point you wrote [X].)
That was my favorite pen until I tried the Pentel EnerGel. It has gel ink, like the G2, but the barrel is slightly fatter (thus easier for me to grip). The "killer feature" of that pen though is the pocket clip automatically retracts the point inside the pen. I've had a few shirts ruined by forgetting to retract the point before clipping it to my shirt, and this pen does that automatically. When you push the "clicker" on the end to extend the point, a little plastic tab gets pushed down and wedges under the end of the pocket clip. As soon as the clip is slightly lifted (or a piece of fabric gets shoved under it), the tab is released and the spring pulls the point back inside.
I got it mainly because of the looks (most fountain pens tend to lean towards the ornate, if you get what I mean, while this one was exactly the kind of design I loved in matte black, exactly the color I wanted), the weight (since I have tremors, heavier pens help with writing) and the price (less than $100).
Since what everyone will be looking for in a pen will be different, maybe you could look online...but if there's a store near you that sells lots of different types of fountain pens, I really suggest going in person to try out a few, and to talk about what kind of pen you had in mind.
When I went to Flax (a local store in los angeles) I went there with a couple different pens I saw online in mind, but trying them out in person, I wasn't satisfied. I spent a while talking to the guy helping me, really stressing the weight and price point as being my main criteria for selecting one, and he pointed out this little beauty I didn't even know existed.
I like the G2s, I even still have one around for when the fountain pen would be inappropriate to pull out (one being my favorite color was green, so I got a dark green ink for it that I usually use over black), but this makes writing fun again :)