The good; The OCA (community association), the people (both community and Odoo), the basic framework gets you up and running fast. You can tell it’s grown over time, and it’s got sharp edges, but you can get what you need done. Not necessarily in the prettiest way. The backwards compatibility and desire to not break the core APIs is generally good from what I’ve seen so far. Individual modules depends how much Odoo themselves lean on it afaict. GitHub access for partners and direct source access to both enterprise and community editions has been extremely helpful.
The bad; imho testing is a pain. The ORM uses Polish notation to build filters, which if you’re used to SQL is frankly irritating to work with. The ORM itself is quite clever, but it’s also not like any ORM I’ve worked with. The dev docs aren’t great, beyond the basics. The quality of modules in the “App Store” is extremely hit or miss. Odoo official “support” as a partner is questionable. I feel like they’re under pressure to get you to pay up to be a partner and then some period of time you might get help later. Anecdotally I’m led to believe our partner account manager has been pushing us hard to host a local event at our own cost (I’m not that involved with that side). The last few versions have seen more accounting features drop out of community edition. Some of the official apps are basic.
Is it better than SAP, Dynamics, etc.? Probably not. Is it good enough given the price point and flexibility, for smaller businesses? Probably, especially if the business has been tying together lots of apps adhoc.