I am a big fan of the almost golf balls. The big pros are you are hitting something that actually looks like a ball (unlike the "birdie ball"), the ball is dimpled so your swing flaws (slice, hook) will show up, and it is pretty safe. The big cons are that while it can be used for judging distance, it's 1 yards of "real balls" to 1 foot of "almost balls" isn't perfect; it doesn't have the mass of a real ball so if you hit out on the toe or heel it doesn't cause the club to rotate like a real ball would; also because it doesn't have the mass of a real ball, on a windy day it is almost comically thrown around.

With all the pros and cons, ultimately, if it helps you get outside and hit balls more often than you otherwise wouldn't have, then it is probably good.

As an alternative if you have an even smaller yard, Golf Digest makes similar balls that are sold in Target stores that are dimpled like the almost balls, but have even less mass and have an even shorter total flight distance.

I do like the foam ones mentioned by b-n-r also, and I also use them for indoor swings.

I really do not like the wiffle balls or the balls that are nothing but a plastic shell. I find that the plastic breaks way too easily and then all you can do is throw them away.

My personal ranking:

Almost balls > Golf Digest balls at Target > Foam balls > Wiffle Balls & Plastic shell balls.

Nothing is better than real balls, really, but if you are constrained by real estate, getting out and hitting something and getting the feedback from a ball flight is better than nothing at all.