It's not really a budgeting app, though.
For us, we like that the budget is tangible. There is only so much money we can withdraw from it, which helps us avoid blowing through our budgets. It also works well with the way we split costs. I make more money, so I put more into the pool every month, but other than that we don't need to think about it.
The nice thing about manual entry, and using Splitwise, would be that you could pull out your phone and say "your turn to pay" and treat it like a balancing act instead of transferring money. Whoever is behind, pays today.
Used it on a couple of trips and thought it was fantastic. No sign up required, free to use, and open source. Worked well for my use cases (trips, not month to month reconcilliation) so YMMV