There's a lot of discussion here about radar range and it's usefulness that IMHO seems a little detached from reality or at least from the typical reality in the PacNW. In practice, as was stated above, most small boat radar will not see more than about 6-8 miles as it's line of sight and not mounted that high. So that idea that a 15 mile range is somehow useful is, for the most part, nonsense. The rest of the comments below apply to mostly typical small boat radars and not the very high end, higher power, large antenna radars that the occasional sport fishing boat purchases.
First, if you're really concerned about seeing those large vessels (tankers and cargo ships) at a greater distance you're much better off having an AIS receiver on the boat. All vessels over 300 gross tons, all tankers and all passenger vessels over 150 tons are required to transmit AIS. With AIS, you will not only see those vessels at much greater range, the AIS system will tell you the speed, direction of travel and usually the intended destination.
Second, as far as seeing weather on the radar goes, the practical experience is that you can generally see it as well with your eyes as with the radar since the range of the radar is only 6-8 miles. Once you are in the weather, you can't see outside of it due to the excessive rain clutter and your ability to see very far within the weather is further limited.
Third, while the radar will show you the outline of land features, the map on your chart plotter provides far more information about the land (and underwater contours) and is far easier to interpret. So IMHO, the idea that the radar is a useful navigation aid is also nonsense. With a decent GPS and a backup/secondary GPS (hopefully at least one that is battery powered) + a depth sounder, the navigation is covered.
IMHO, the best use for small boat radar is relatively short range, high resolution detection of boats in the fog. I typically have my radar on when it's foggy out and when visibility is limited, I'm moving fairly slow. I'm often fishing in such conditions and there are often other small craft in the area. A typical range setting I use in such conditions is 1-2 miles as that provides the resolution needed to weave between targets. Occasionally, there are sport boats that are zooming around at 20-30 kts in such conditions and my short range will only give me 1-3 minutes notice of a boat coming my way so a high level of attention and diligence is required. In somewhat less foggy conditions or less crowded fishing, I might use a range as large as 4 miles but I can't recall a time in 100's of hours of radar use where I got useful information from a larger range scan.
One thing to realize is that larger range scans generally reduce the size of the return signal display on your screen. With a typical 8-10" (occasionally 12") screen, the return signals for long range scans can be quite small. Also, the rocking motion of the boat in anything but the most calm sea state, will often result in a target only being picked up on percentage of the scans. Often times, the difference between wave formed clutter and a real signal is the occurrence of the real signal in the same spot repeatedly. Shorter range scans make it easier to pick up the smaller boats since the size of the return on the screen is larger. So IMHO, the primary useful range of small boat radar is most often in the <4 mile range.
