My favourite situation to play uke is in a string band. There's always guitar players, and the uke spreads out the rhythm sound and gives everyone room. Uke and guitar together sounds better to my ears than two guitars or two ukes.
I'm currently putting a string jazz band together that's either violin, uke, guitar, and bass or, if the viloin isn't available, we'll add either a (tasteful) drummer on brushes or a second guitar. I miss the fiddle, but when he's not there I play all the heads, which is also cool.
When I'm playing with a guitar player, I always watch the left hand - if they go high, I go higher.
In the swing group, sometimes we play the four-beat pump rhythm together, lock-step, and sometimes I play shots and accents like a piano player. If the guitar is playing fairly straight chords, it gives me license to play weird inversions and substitutions up the neck. When the guitar or bass is soloing, I play plain and steady, first position, and quiet; usually I just brush with my thumb up over the neck. It's good when the guitar player understands that they can solo in the low register, rather than being up where I am all the time. If they're used to playing with another guitar, they tend to think of melody-land as starting around the seventh fret. That's my turf.
I also play in a duo with a guitarist/singer. Usually, when he is playing a guitar part that's got a strong low-end bassy sound, I'll play uke. When he plays a higher part or uses a capo, I'll play bass. It's a question of where the "room" is. When he plays mandolin, I play guitar.
With any band experience, the important thing is to listen to what's going on and fit into it, find your musical space. I'll play very differently with another uke or mandolin (that's interesting) than with a guitar or bass.
