I've always liked the Apple Newton. Conceptually, I think the Newton was right on: it was a very user-friendly device. It was built on great hardware. It was very versatile. I was upset to see Apple drop this great product.
My first Newton was an OMP (the Original Message Pad). It actually belonged to the executive director of my department at Penn, who reportedly used it for less than a month before he decided that it was junk. I agreed: the original message pad was junk. Conceptually good, but the hardware wasn't fast enough for the under-developed software.
From there, I went to the MessagePad 120. This was a much better piece of hardware, and was fairly mature - except its lack of internet communication ability. I think that this was probably the biggest problem with the Newton; it was designed before normal people had access to the Internet, and therefore wasn't very well integrated into Internet life. The communications architecture of the entire Newton line is fairly slow to begin with, and the TCP/IP support came too little too late. If the Internet had been more prevalent when the Newton was designed, it might have been a more compelling device.
Putting aside the MP120 in favor of an Internet-capable Newton, I got an MP2000 ("mp2k"). Unlike what Apple was doing with their Macintosh line at the time, they got this Newton right on the money: more RAM and a processor that was an order of magnitude faster. They also made it bigger in an age when people were talking about how to make PDAs smaller - a gutsy move. I think it was warranted: the Palm market was fairly well seated by this time, and I think it would have been a mistake to put the Newton OS against it.
After upgrading my MP2k to an MP2100, I found my first - and only - flaw with the MP2k hardware. The serial port used an adapter in order to plug the keyboard in. I took my MP2k on a business trip with me, and used it for email. While there, the weight of the dongle pulling down on the serial port ripped the connector off of the motherboard. I took a quick look inside, and realized that there is absolutely no strain relief on this connector - it's surface-mount soldered on to the motherboard, and the weight of the connector is apparently enough to eventually pull the connector free.