Well, I use Download Accelerator. Its prime function seems to be the ability to resume, and have multiple downloads going at once, which are both handy, and I haven't noticed any major spyware problems. I always reject anything from Gator as a matter of principle.
But surely these things are overrated? I mean your download speed is limited by your connection, isn't it? Surely a 56kbps modem can only download at a maximum of 7KB/sec? If I have four things downloading at the same time, all other things being equal, they will each be downloading at a quarter of the speed that one thing would be. No? 'Tis my experience anyway.
A useful product, but certainly not worth paying for.