I have the KM100R with string trimmer and the pruner. I love it. There are some things you should know before buying.
First of all, it does not have the explosive acceleration of a 2-stroke. You have to give it a 1/2 second to get up to speed before laying into the weeds. Once you get used to this it becomes second nature. On the plus side, it doesn't bog down easily. The string will lay back long before the engine will slow. If the string grabs something good, there is a clutch, but it can be a jolt before the clutch slips as all that torque comes to bear for a moment.
Second, the pruner makes a darn good chain saw on the end of a pole. I would put the power as better than any electric consumer chain saw and as good as most small consumer 2-stroke chain saws, or better. Don't worry about power for hedge trimmer, it's there in spades. In this respect, the torque makes it a better pruner and hedge trimmer than string trimmer.
Third, it is a bit heavy and big. If you are a homeowner you can use the size and power to finish quicker. If you use it to make a living all day long, you're going to develop big, strong arms. If you alternate its use with other tools it'll probably be OK.
Fourth, I bought it at the beginning of the season so I can't tell you the long-term reliability of the split shaft. However I have gotten the pruner saw stuck in branches, and never had a problem with it seperating or shifting. However it is REALLY NICE when that happens, to disconnect the power head and not have the weight bending the saw while you work to free it.
Fifth, the bump feed actually works, it starts and runs reliably... you know, it's a Stihl.
Don't let the Ryobi 4-stroke blues get you down, this model has no oil sump or pump, runs fine upside down, sideways, etc.
Sixth and finally, you don't have to buy another 110 power head for your backup. There are two smaller/cheaper models in the lineup that work with the same tools. For a Pro Lawn Guy, two power heads, two string trimmers, a hedge trimmer and an edger (or whatever) and you're looking at serious productivity for two workers.