Something that hasn't been mentioned - the POD, J-Station and V Amp are amp simulators that also provide basic effects (generally Compressor, Flanger, Chorus, Phaser, Rotart, Echo, Delay and Reverb). The Boss GT-3 and GT-5 are effects processors that include amp simulation and have many more effects. The POD and J-Station are designed as direct-recording tools and, as such are more difficult to use in live situations unless you buy the (expensive) foot-controllers or use a midi controller. The V Amp includes a two-button foot switch that allows you to switch between 5 presets in a bank. The Yamaha DG-Stomp provides 4 foot-switches that allow you to set tap-tempo/tuner and toggle between the 90 user/90 preset patches (you can also set the patch toggle switches to control effects on/off (modulation, delay, reverb). If you have a kickin' amp setup, you might not need (or want) the amp simulators.

I ended up buying the DG-Stomp for the sounds and the ability to use it out of the box as either a direct-recording tool or in live situations. I use the Stomp with a Rolls Midi foot controller to select patches and set the built-in foot-switches to control effect on/off.

Another consideration is noise - most of the amp simulators include a noise gate since many digital systems generate a lot of noise. The DG-Stomp doesn't have one, doesn't need one.

Our lead guitarist started with a GT-3 and had a chance to pick up a GT-5 at a great price so he upgraded. He was able to get some really good sounds from both, although he does have occasional noise problems. He works at a music store and can get most of the effects pedals at much reduced cost, so I always listen when he's talking about gear. The amp simulations on both the GTs and the Stomp are great! Both of us regularly use the pedals without an amp, just running through the PA. He sold his Mesa-Boogie and always runs through the PA. I sometimes run through my '65 Super Reverb, but I never move it around anymore. It always stays at our main gig.

I still wish for some of the effects that the GT-3 has - the acousic simulator, single/humbucking emulation and pitch shift, so I may invest in a GT in the future.