I have the Quantum 2. It's OK, there are occasions when the Doppler feature lights up some thing which might otherwise go unnoticed if you weren't paying attention. If upgrading systems or installing new, sure I'd do it again. But if I had a perfectly functioning Raymarine from last generation or even two back, not sure I'd jump at the chance to dump the cash. It isn't a huge leap in function. The ARPA is a little more solid in tracking when conditions are marginal. The target discrimination is better but not night and day. At close range is when you notice it most. It draws considerably less power which is usually not a problem on a motor boat. It powers up more quickly from idle, which is nice.
I had the last analog 18" on the sailboat, replaced it with the HD digital, a small improvement, and now the Quantum 2 on the power boat, another small improvement. I may replace the sailboat one again just because the weight and power are so much less. Be aware that it does not work with older Raymarine chartplotters.
The ARPA function would be nice but if you have land in range it confuses it to the point it has to be turned off.