You are not going to break any speed records but the 150 is a very reliable economical engine. I have a 2120 07 and have close to 900 hrs on it. I've been out 60-65 miles and trolled all day only burning approx 65 gallons. I have done several trips at this distance and the burn has been consistent.
I have a 2120 with a 150 hpdi (2 stroke). I can cruise at 30 mph no problem. The boat planes nicely. I have a pulled a kid on a tube with no problem either. The boat is not designed to be a speed demon but it moves very well. I havent spoken to anybody that is dissapointed with a 150, and when i see other 2120's on the water they all have the 150.
Are you looking at a brand new one?
I have a 21DVCC with a 2stroke 175. I can hit 43 on GPS and the boat does not feel under powdered. All things being equal I'd prefer a 225, but don't "miss" the extra horses enough to do anything about it....
I have a 2120 with a F150 4 stroke. I hit top end with a half tank of fuel and just me on the boat at 35 MPH (GPS). It is not quick out of the hole but I have trim tabs and I can get up on plane pretty easy. I've only felt under powered one time at the mouth of the Chesapeake bay with a good wind and outgoing tide. I would have preferred a little more power but didn't feel out of control at all.
I've had some talks with a couple of Yamaha mechanics that said they fel the 150 was the most reliable motor Yamaha produces. I have 800 hours on mine and it has been great. Nothing but regular maintenance.