I like Susan Polgar's Chess Tactics for Champions. I have nothing against chesstempo or Tactics Trainer per se, but I agree with fightingbob that when someone is just starting out, they might profit from a book with a bit more explanation, so that they can learn the basic concepts, and have a better shot at solving the puzzles. Ward Farnsworth's book has some really nice explanation, and is well regarded for tactics.
Of the books mentioned by the original poster, I do think Silman is worthwhile. His imbalances concept does help you set goals.
I also think reading books generally is a good thing. Also libraries can help you get books written by better authors on better quality paper without costing you anything.
after reading this i checked it out on Amazon - the first 80 pages of Chess Tactics for Champions are available when u click on "Look Inside" and i have to agree with the 'jam... i liked it so much i bought it... $6 USD used.
i read further up this thread that fightingbob has 1000 books... so jst thought u guys would get a kick out of this cuz it is my first chess book... actually, my parents gave me one for my B'day but this is the first one i eva bought :)