I'm thinking about an undersink water filter and have a few questions:

1) are they worth it?
2) do they work?
3) any recommendations or advice on what to avoid?

thanks in advance
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Most definitely worth it for the clean taste and peace of mind.

I stand by the omni-filter 2 cartridge filter under my kitchen sink.
The output from the filters goes to a tee that feeds the fridge's ice
maker and a sink mounted filtered water spout. I change the filters
every 6 months (I just mark my calendar). It's a great setup. However,
if I were to do it again, I would have gone with a whole house dual
filter system to protect the entire water run in my house.

The model I have is the OT32 Series A:
http://www.omnifilter.com/undersink.htm

The filter combination I use is GAC1 for the first filter, then CB3
for the second filter:
http://www.omnifilter.com/replacement.htm#Undersink

I order the filters in six packs from either FilterPros.com or
Omni-filters.com depending on who has the best overall deal at the
time. By ordering 6 packs, I have 3 years worth of filters on hand,
and save a bunch of money over buying them at Lowes or Home Depot:
http://www.filterpros.com/newsite/products.php?CMP=4
http://omni-filters.com/cartgac1.htm
http://omni-filters.com/cartcb3.htm

Hope that helps.