You were so right. I used it today for about an hour. What a difference between the Poulan and the Stihl. The Stihl cut through the buckthorn like it was butter. I cut down about 7 trees, three of which had diameters of 6" or more at the cutting height. After cutting each down, I also chopped up the trunks and branches into about 6 foot lengths. All of this would have taken about 2 hours with the Poulan, and I would not have been able to do it in one outing (because it would have run out of gas or just quit on me, and I would have had to let it cool for a couple of hours before I could get it restarted).
Also, after about an hour, I decided I should stop to give the new engine a break. I took it into the garage, cleaned it up, switched the bar around, added gas and bar oil, and started it up. It started in four pulls, and didn't quit on me. With the Poulan, I could not for the life of me get the thing to start up once it had been running for a while and then quit. That can be kind of dangerous, too. Often I would need to re-tension the chain as it expanded during use. I let it get way more loose than was probably safe because I didn't want to turn it off, and didn't want to risk tightening the chain with it still running (even more dangerous). With the Stihl, it doesn't look like I'll have that problem. The tensioner is a little plastic gear that can be adjusted very simply with my finger. No tools, no taking anything apart.
Surprisingly, the Stihl is also lighter, yet seems to have quite a bit more power than the Poulan. I guess you get what you pay for (Poulan was $99 and the Stihl was $199).