As soon as you start to do calculations on doubles you may get unexpected rounding problems because a double uses a binary representation of the number while the decimal uses a decimal representation preserving the decimal digits. That is probably what you are experiencing. If you only serialize and deserialize doubles to text or database without doing any rounding you will actually not loose any precision.
However, decimals are much more suited for representing monetary values where you are concerned about the decimal digits (and not the binary digits that a double uses internally). But if you need to do complex calculations (e.g. integrals as used by actuary computations) you will have to convert the decimal to double before doing the calculation negating the advantages of using decimals.
A decimal also "remembers" how many digits it has, e.g. even though decimal 1.230 is equal to 1.23 the first is still aware of the trailing zero and can display it if formatted as text.
