Originally Posted by squodge
Yes, you absolutely need Pro instruments to get the pro achievements. How would you play chords on the plastic guitars lol? For pro drums, you need at least 1 cymbal - but it's great fun to have all 3.
The cost is very low if you're an avid RB gamer. I certainly wouldn't recommend anyone buy the instruments unless they expect to play the game a lot, in which case $300 is very cheap for what the instruments are.
Don't forget besides having the instruments you need to have some skill, or a shitload of time to learn how to actually play the pro instruments.