First of all, let's clarify the differences between assessed value, appraised value, and market value:
Assessed Value: Value placed upon a home by the town assessor for tax purposes. These numbers are created and used to raise the tax revenue used by a town to operate.
Appraised Value: The value assigned to a home by a licensed appraiser, usually done when a mortgage is being created and the property is being pledged as security for a loan. May also be done electively by a homeowner for a fee.
Market value: The price a ready willing and able buyer will pay for a home at any particular moment in time.
Clarifying which value to which you refer, I wouldn't focus so much on assessed or appraised value as I would market value, which can be established by studying the most recently sold comparables in your area.
If the home is 15% below what the most recent comparable home sold for, it could be a good investment, especially if you plan to hold for 5 years.
Without more information, it's hard to advise more thoroughly, though.
Hope this little info helps, at least.