I've been using the disposable fiberglass filters for a while now.  They catch a lot of dust... last about a month or so.

I've been thinking about moving to the washable filters, so I don't have to worry about running out of filters or remember to buy them.  It seems like the electrostatic ones are a little more effective.

I'd like to keep it low-cost: I don't want to spend a bunch of money on a filter, especially throw-away.

What type of furnace filter do you use?

disposable.  And you're supposed to replace them every month?  

In my experience the residential electrostatics are NICE.  But personally I buy pleated filters in bulk from either Grainger or the local HVAC supply house.  Affordable reusable filters aren't that effective.

Step up to a pleated filter for a 3 month period and you'll eb pleasantly surprised when compared to a fiberglas filter.  I modified my home unit to accept 2" filters as opposed to the thinner residential filters.  Surface area is much greater than the thin ones.  I can go 6 months to a year on one filter.