I won't go into looks and such, thats a personal preference. I will however, touch on a couple of technical pros and cons.
Pros: Nutrient exporters. They can help greatly decrease the amount of nitrates in your tank. Using nitrates, CO2 and other such things, plants will convert all of this fish waste matter into plant growth and O2. Both pretty good, as you can sell plant clippings, and O2 doesn't hurt fish. Several people put plants into their sumps to help reduce nitrate levels within their show tanks.
Cons: Can cost a bit more, but if you are willing to look around, and do a bit of DIY work, it won't come out to be much more expensive. Floating plants don't require substrates, a few plants only require low/moderate light tanks, etc. Once you get into the domain of CO2 injected, high nutrient tanks, you do end up doing a bit more work. You will need to do weekly trimmings, bi-daily dosing of nutrients, etc. However, with good planning and such, that kind of maintanence can be kept down to a minimum. Currently, my plant tanks only require 10-15 minutes more maintanence time than my fish only tanks. From what I've experienced, plant tanks are a bit more time consuming initially, but as time goes on and the tank matures, maintanence time will drop. Nutrient dosing will become as regular as feeding your fish. Some people will say plant keeping is difficult. While slightly more involved than keeping FO tanks, if the research, reading, etc is done ahead of time, a lot of the headach many plant newbies experience can be eliminated.
HTH
-Richer
