Tantalum capacitors have a couple of important design advantages:-

• They can be excellent at removing noise (for instance placed across power rails) and improving the EMC performance of a circuit.
• They are physically small

The main downside is that they are more expensive than say an equivalent electrolytic capacitor.

Voltage

It is important to derate tantalum capacitors voltage by 50% (e.g. if placing across a 0V and +5V supply rail select a 10V or higher rated tantalum capacitor).  The reasons for this are complex, but it was discovered when first developing the capacitor that it needed to be used for a significant period of time to reveal any faults and their life was greatly extended by derating down.   This is one of those rules you should just follow as a designer to ensure the longevity of your manufactured hardware.

Faults

Tantalum’s can cause really strange problems if they fail or are inserted the wrong way round.  If you’ve got something very odd going on its often a good idea to check and even try removing tantalum capacitors to see if they are the cause.