There is tons of work in Madrid right at the moment, rather surprisingly that in spite of the fact that English is the main second language taught (and has been for the last 10 years) demand has increased rather than decreased. There is currently more work than teachers. As for the specific question of whether getting your TEFL qualifies as a job offer (in relation to getting a work visa) no it doesn't but then it doesn't really matter as half the teachers in Spain are Americans and most are paid in cash. The money is quite good but there are a couple of negatives, one is the cost of housing, if your single living on a teachers salary (usually 16 an hour in the hand 20-24 hours a week 1200 to 1500 take home) it will probalbly means renting a room rather than an apartment and secondly their is limited work during the summer, so you need to plan for that. But inspite of a few minor negatives I think teaching is the greatest job in the world, your paid to talk with people (half the time very cute Spanish girls

) and you usually never work for more than an hour or two at a time, I go home as everyone is else is beginning their work day. It's nice enough that Chris is serriously thinking of joining me. While I dont' think we'll do that in the short term it's something we've seriously talked about.
Is it worth it, well I'd put it this way, if the choice is Hartz IV or returning to America than I'd say go for it
If you have any specific questions feel free to pm me.
Rob
edit: for General ESL stuff go to daves ESL cafe (under jobs and Spain) and for Madrid Specific check out
MultiMadrid