THey do work, but have a very low (3db per octave) roll-off. What this means is that they let a lot of lower frequencies through. Most amps use a 12 or 18 db per octave roll off. I ran a set of 200 hz ones on my dash speakers in my old truck. Worked good, but they aren't the greatest crossovers (stopped my 4x6's from buzzing on bass hits though).
I needed a pair for my Rockford Fosgate dash speakers, so I took the info that I needed to two different Radio Shacks. The first one looked but couldn't find anything like I needed and the second one sighed and said they used to sell stuff like that but don't anymore. ~get with it RS!~
I ended up buying on Ebay for $11.49 including shipping from this guy: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3062175755&category=14931
The 600 Hz blockers work great. They do exactly what I wanted them to do.
Just make sure you know what cut-off frequency you want before you buy!