Indonesia is one of the game's most distinctive civs. It has a wide array of situational (but potentially powerful) uniques, and therefore demands a great deal of flexibility to play effectively. At times, you'll need to abandon familiar strategies to unlock the civ's bonuses. At other times, you’ll need to ignore those bonuses, accepting that the cost of exploiting them outweighs the benefit. I won't pretend that Indonesia is one of the game's strongest civs, but if you like the idea of a civ that rewards flexibility and creativity and can end up playing very differently from game to game, it can be an incredibly rewarding civ.
Indonesia’s UU, the Kris Swordsman, is a frequently maligned but potentially very useful unit. While a couple of their unique promotions are negative (two of the eight possibilities), most are positive, and the best ones are absolutely amazing -extra movement and attacks, a massive defense and healing bonus, full heal on kills. These units can be upgraded and serve as the core of your army for eras to come. Units with the negative promotions won’t be worth upgrading, but their penalties are not by any means crippling, especially if you adjust your tactics to account for your units’ varying capabilities. The utility of Kris Swordsmen is restrained by the high costs of early game war and the uncertainty of spawning near iron, not by the possibility of getting bad promotions
Indonesia's UA- Spice Islanders- can also be very powerful, but there's no guarantee that there will be suitable islands anywhere close enough to you to be defensible, and you won't know until you've built triremes and possibly caravels to find out. Also remember that even a nearby island will require optics to access, which probably means either altering your early game tech path or delaying your colonization while you set up mainland infrastructure. That said, when you have the chance to do so, settling islands as Indonesia is a huge bonus- each city with one of their unique luxuries is essentially a free city happiness-wise. You just need to be willing to deviate a bit from "standard" strategies like 4 city tradition or 6 city liberty.
As flashy as unique luxuries and Kris Swordsmen are, though, Indonesia’s most dependable, and probably most important, bonus is its UB, the Candi. Figuring out how to use it, though, can be something of a puzzle (a puzzle that may not have the same solution every game). Other faith-oriented civs receive relatively small, very early bonuses, providing massive boosts towards founding a religion, and becoming less significant as the game goes on. The Candi, in contrast, comes too late to help you very much when it comes to founding a religion (you’ll almost certainly want to find some other means of doing this, though), but its bonus is huge -you’re essentially guaranteed +4
(2 base and 2 from the city’s dominant religion), and you’ll frequently get +6 or even +8
even without going out of your way to do so. With religions already founded, though, it’s up to you to find a way to turn this bonus into something that actually helps you win the game. With a good source of iron and hostile (or vulnerable) neighbors, that could mean using Holy Warriors to build an army of Kris Swordsmen. With a well-established rival religion nearby, it could mean spamming missionaries with Interfaith Dialogue. It could mean trying to spread a key founder belief across the world or exploiting a reformation belief like Jesuit Education or Glory to God. And whatever else you do, once you reach the end game, the Candi should ensure that you have more faith for great person buys than anyone else, giving you the edge in a close science or culture race. Faith in CivV is an amazingly flexible resource, and with the right planning, you can ultimately use it to accomplish just about anything. If figuring out how to do this (while also juggling a situational UA and UU) sounds like a fun way to play civ, Indonesia is a great choice. If you’d rather have something that pushes you into a specific, dependable, strategy, you'll probably be better off looking for a different civ.