There was a time when I (no, it's not gonna be "cryin'", hohoho) hardly could tell the difference between a tube and a transistor (which all digital-amps and pod-like effects are) amp because I didn't care... that's about a month ago (what a shame - I've been playing guitar for nearly 7 years now).
I was quite interested in the flextone, which is basically the same as the pod + a speaker. wow, it's nice... you got something LIKE a marshall-tone, something LIKE a vox. But hey, it really doesn't sound the SAME. I'm not the type of guy who says any strat that doesn't sound like a fender sounds shitty - no, if it's got a nice character it's ok... but to come back to the pod: its tube imitation is quite bad - even I did hear that. that guy in the store plugged a low-end strat into a 500-bucks-tube-amp and squeezed those sounds out of it that I've been searching for. with a pod you have to get the power from the distortion; less distortion means less power. with an average tube amp you get so much blues-power out of the cranked up clean-channel, damn... that was an amazing demonstration.
why should one buy a pod? because you want a wide variety of sounds. but you can throw away 2/3 of it, because the clean and crunch sounds suck. it's only good for heavy distortion... so there's not much variety anymore.
I doubt about using it as an effects-box for a tube amp also; this way you cut off
the tubeamp's pre - and what comes out is half an eunuch.
and by the way: doesn't a pod always simulate an amp AND speakers? so what you get is guitar-> simulated amp -> sim. mic-ed (and polished) speaker -> real amp & speakers... does this really sound good?
anyway. I like digital stuff because it's cheaper and - as I said - any effect used with a tube-amp will cut off some of its character. there's some digital amps of the big brands that don't imitate but create their own "tube-sound" with built-in multi-effects in the right place that don't kill the sound... quite good, really. I ordered one ;-).
... and if you use your pod for some demos only: you could have saved some money by buying a 'real' multi-effects-pedal with some amp/speaker-simulation (boss, for example).