I worked with the 3100 a lot. Both the PS and photo versions.
I had no mechanical problems at all.
I have a 3200 24" photo now.
I have had ongoing mechanical problems. Last week they found the paper path sensor to not be seated correctly in the plastic holder. It seems to be working on all media now.
The build quality is not stellar. While some parts are for high MTBF ratings , others are cheaply and poorly made.
Image quality is sometimes above Epson, sometimes on par , sometimes lesser. Globally the Epson will be slightly higher.
The new reds solve all red problems for matte paper and make better skin tones than previously. Personally I would only consider the 3200 for that reason.
The other advantage of the 3200 is all the mechanical parts were updated to correct for some minor problems on some 3100s.
So the 4880 or other Epson will be a high quality build, easy to load, and has aspirated platen to hold paper flat without rollers. This also lets users adjust platen height, and run very thick media. Epson dark saturation is and always has been better than both Canon and HP.
The HP Z series rarely have a noticeable clog, and when or if they do it's easily rectified, and inexpensive to do cleanings. Running costs on the Z are very reasonable. By having a separate matte black head, switching papers is a big bonus as it is fast and doesn't cost anything. I have had very prompt courteous tech support in France.
Oh yes the Z series have inks that are actually at the top of their type in permanence and stability. There is a gloss differential with the light grey ink but is suppressed with Gloss Optimiser and is not that apparent on most papers.
So if moving from Epson to HP, just make sure your gains are more than your losses.
With all the recent stumbling on the Mac platform with CS4 and Snow LEopard with printing no color management charts for profiling, you can see the advantage of the built in spectro which leaves you independent of the system or app level pains between upgrades.....