A ukulele is more like a guitar than it is like any other instrument. If you consider yourself a competent guitarist, I will project your understanding of the guitar onto the ukulele on the next page.
A guitar has a lot more range than a ukulele. A LOT more. A guitar goes down, down, almost 2 full octaves from where a ukulele can go. Playing a guitar after playing a uke - is like joining up with a bassist. It's also a bit like pinning yourself under a bulky piece of furniture.
Ukuleles are less expensive. Ukuleles are much easier to transport - I sometimes like to 'conceal' a uke in my jacket, it seems to magically appear in my hands. Ukuleles require much less strength and toughness-of-fingers than a guitar. They are a bit easier to understand - less strings means less notes which means less things for your mind and fingers to remember. It's not much less, but if you assume your fingers have a 5-fret spread, the math will tell you that you only have about 4% as many options on the uke as you have on the guitar.
Compare and Contrast: Ukulele vs. Guitar! (Click the green arrow to hear them back-to-back)
Perhaps the biggest difference between a guitar and a ukulele is in the effect that it has on people when you play it. Everybody has seen lots and lots of people strumming a guitar. Some associate the image with tall, cool, hip people being rich and famous - some associate it with a singing junkie on every streetcorner. A ukulele does not carry these 'mundane, everyday' kind of images with it. Ukuleles are much more likely to make people smile. Whether it is surprise, novelty, cuteness, a lack of pre-conceived notions, or a combination of those, there is a certain freshness to a ukulele that a guitar, in it's overexposure, simply does not have.
Lastly, I feel I should point out how, if you are a singing entertainer, a uke brings you closer to your audience than any other instrument. Singing from behind a piano is very isolating - singing from behind a guitar is much, much less so. The audience gets more of you, and your singing becomes more the focus. A ukulele takes this relationship even further - there is almost nothing between you and the audience. Your singing becomes even more focal - and you have to work for that, because with so few notes, all being high notes, a ukulele gives you a bit less to fall back on. But if the singing is primary, and the accompaniment secondary, you can use all that to your advantage.