From this thread on the RB forums: http://www.rockband.com/forums/showthread.php?t=210789
which is a pretty good thread (although I disagree that five-lane keytar is nothing like pro keytar; I think there is definitely value in getting good at the five-lane mode as there are some shared attributes) Anyway, from that thread:
I've had about nine years of formal piano instruction, years and years ago, but I haven't played in a long time. I picked up my keytar last week and dived right into the tutorials. The easy ones are easy, then they ramp up quickly and I got frustrated. So, I went in and dived into Expert Pro and tried some songs. Wow. HARD. Really, really, really hard. Got more frustrated and discouraged.
At this point I was worried because I don't really want to spend the time practicing each song. I don't mind practicing some but this was really brutal, just failing out over and over again and flailing around feeling completely lost.
So, I played some regular keys for a while, which was fine. While playing regular keys I realized there were some technique issues that were causing me problems. The most important one is I tend (like a lot of people) to play a little more legato, and RB does not like that. Also, sometimes when moving to a different chord, with a note or two shared between the chords, I would leave the keys that are shared depressed and RB doesn't like that either. Cleaning up these issues helped my 5-lane keys results.
Anyway, I took a little break over the weekend and then said "what the hell" last night and dove right back in to Expert Pro. MAJOR improvement. No, I'm not brilliant (to say the least!), but I was able to sightread a 4 1/2 stars on Rock Lobster (91%) and to pass a bunch of songs (with crappy scores) that I was failing out on before. I am overall doing much better, although there are still some songs that pretty much fail me instantly because I'm not up to fast three or four note chords yet. And let's not talk about the really crazy high-end songs. BUT, I am now sight-read passing a good number of songs on Expert Pro, and that means I can play a number of not-the-easiest songs and learn them by playing, which is what I really wanted.
The difference for me last night was that it was really HARD, but it was FUN. Prior to that, it was really HARD and incredibly FRUSTRATING. I think there are some factors that came into play, at least for me:
#1: They keys are small. I'm used to larger keys, as I would imagine most people with any previous experience are. It took me some time to adapt to the size, which is very important so you can reach accurately without looking down. [EDIT] A poster subsequently mentions that the white keys are regular size, but the black keys are in fact narrower.
#2: Unlike Pro Guitar, keys only light up when you depress them. While this makes sense, it can make it very difficult to position your fingers correctly without looking down. Having previous piano experience here helps a lot, I think, as well as simply playing the thing enough to feel comfortable with the distance between keys.
#3: It takes time to even begin to be able to track RB pro keys notation on sight. I had to really let it sink in before I started feeling at all remotely comfortable moving around on the keyboard without either pushing keys to see where my fingers were (breaking combo and dropping your crowd meter of course) or looking down at the keys.
#4: Playing as a keytar. One of the things I chose to do was to play it as a keytar, not horizontal. That definitely feels awkward at first, but I feel a lot more comfortable now after playing a good amount of 5-lane keys.
I don't know if this is helpful to anyone, but for me, being patient and playing some 5-lane keytar has really helped me feel more comfortable with Pro Keys. I feel a lot closer to where I expected to be: "played a lot of piano a long time ago and is really rusty" rather than "what is this thing I have in my hands and what the hell is that on the screen and why am I failing out over and over again?".
td;lr version: It takes time to learn the dimensions of the keyboard, and it takes time to learn to sightread the RB notation. For most of us this is the first time we've had to learn new RB notation since RB1 (for some even before back to the GH games) and we are used to just knowing what to do.
Also, I think this will benefit me when my Squier (just pre-ordered!) comes; I am not as rusty on the guitar, but I know now that it really will take me some real effort and patience to learn to sight-read even simplistic Pro Guitar charts.