I'd second the vote on convenience. And laziness induction. :-D
Somewhat of a negative is that you do need to keep the CYA (stabilizer) level rather high (70-80 is recommended by most SWCG makers,) which means that if you
DO need to shock, you are going to need chlorine in higher quantities.
Depending on your comfort with chemicals, you are also trading handling/adding chlorine for acid, as most SWCG equipped pools tend to have ph creep up. This can be minimized by using borates.
Acid is also required to clean the cell. The cells foul with mineral build up, similar to a hot water heater. High calcium levels can be a problem, as can other minerals, which is why use of sodium bi-sulfate(dry acid, ph down, ect..) is not advised, or over use of cal-hypo(powdered shock) for chlorine addition (I can't remember if use of lithium shock is frowned upon too, as I just use bleach.)
In my pool, I added borates, I usually add 2-3 pints of muriatic acid each week. This year I cleaned the cell once, even though it didn't really need it (my calcium hardness is ~140, in a vinyl liner, PS person keeps telling me "That is TOO low," even though I know that IT IS NOT!)
Most units are designed to be easily added on to an existing pool. Cutting plastic pipe and gluing in the cell is an easy project for most able-bodied people. Hard wiring the electrical power is something that might best be left to a professional, although it is not much more involved then replacing a pump/motor.
I would recommend reading through the forum, and using GOOGLE to see some of the features that users like, or don't like about particular models (there are hundreds of choices, make sure to look for the Australian makers, as they were some of the first makers, with the most copied features((that was a plug to Strannik
)).) One feature that almost all units have, and is really pretty much useless is SUPER CHLORINATE or BOOST( it adds too little chlorine over too long a time to be effective, IMHO.)
Good Luck!!!