When my feeling grew that something was lacking in Aikido, I took up Judo in the summer of 2000 and stayed there, with increasing enthusiasm.
On the surface, Judo is a competition sport with the aim of defeating the opponent by throws, pins, joint locks or chokes. The rules are fairly strict and elaborate; most injurious techniques have been banned, such that Judo is relatively safe, considering it is a full contact martial art. This does mean that the techniques do not work - a well-executed choke means a gentle, but inevitable K.O. within seconds, and after a hard throw onto concrete, the opponent is likely to take a timeout to count his bones.
As opposed to Aikido, which disdains the use of force in principle and demands that the practitioner always stays in a stable, upright position, Judo follows the more general principle of "maximal efficiency". This means that you do what it takes to throw the opponent, but you do it in an elegant and efficient manner. If it is necessary to fall to the ground yourself in order to make full use of your weight, then so be it. If you happen to fall on top of your opponent, that does not hurt (you) either. Quite the opposite: you can use the position to make a smooth transition to ground fighting.
Ground fighting is not intuitive at first. Most people spend the first months rolling from one trap to the next before they figure out what to do (e.g., avoid offering the opponent a stiff arm, don't let them mount you, etc.). However, it is a lot of fun, it is an excellent cardio workout (there are few activities that suck the force out ouf the body as all-out ground randori), and it involves a lot more skill and technique than one would suspect at first glance.
Of course, Judo is influenced strongly by its competition rules and cannot be used as a complete self-defense art. Strikes and kicks and how to defend them are not an issue, and not every opponent in real life will wear robust clothing that can be abused for gripping and choking. On the other side, the techniques are competition-tested, which means that they can work on well-trained opponents and that you can find out to which degree you have mastered them. Furthermore, Judo training inproves strength, endurance, mental flexibility and fighting spirit.