Is "warm" a compliment or a criticism?

Better "warm" than "cold". Better slightly "warm" than hard, steely, over-detailed with pinpoint imaging, tiring over extended periods, etc.

Regards,

I always think of it in terms of temperature, because music is a lot about how it makes you feel: Cold and lifeless, cool and distant, warm and sluggish, hot and irritating; yet, it can also be cool and collected or warm and inviting etc. There's a fine line between the different temperatures/descriptions and I think one thing to keep in mind is that the opposite of warm is not cold but cool. Since our own temperature fluctuates, what is warm one day can seem cool the next and viceversa. So that's another reason to seek a neutral balance (I don't care too much about reproducing what is exactly on the record). I usually prefer my drums to be relatively hot and electronic instruments to be relatively cold, but woodwinds have to be warm (if not either the player sucks, the recording sucks, or my system sucks :P). But this comes naturally. That to me is the whole point of being accurate. You have to screw up to make a warm sounding instrument sound cold.

'Warm' is inherently neither good nor bad. It descrobes a tonal balance that has more energy in the lower half--say, below 500Hz--than in the upper half. (A tonal balance having more high-frequency energy is called 'bright'.)

Warmth is absolutely a coloration, an error, but usually it sounds wonderful if not overdone. But warm becomes 'thick' rather soon.

So whether 'warm' is good or bad is truly a matter of opinion, but WHAT it is is rather commonly accepted.
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Gunbei is right.

Curves=good. Skinny straight stick chick= need sandwich.

For audio equipment, I've frequently heard tubes described as yielding a "warm" or "full" sound. When I listen to my Cary SLI-80 Signature driving my Revels however, I don't notice "warm" so much as "rounded yet detailed".

I think the term "warm" can be interpreted as good or bad- as you stated. So for audio, it's probably a fairly ambiguous and poor term.

At the least, "warm" means that the sound is not shrill or harsh. Most anyone would consider that great news. But beyond that, the meaning seems to depend on writer / speaker and the situation. A system that portrays the beauty of midrange, such as female singer or sax, can be described as warm. But "overly warm" suggests that details are being sacrificed in exchange for presentation of a sound that is pleasant, but lacking in realism.

For many years, it seemed that most transistor gear had a harshness or graininess to it, and so "warm" was a welcome relief from this problem. Todays best systems seem to sound realistic without being either grainy or overly rich or colored ("warm"), and hence this term can had positive or negative connotations.