BMW reliability is good, but even the vaunted 3-series won't be as reliable as many modern Honda/Acuras and Toyota/Lexus, but then again, nothing is. The BMW makes up for it by being better in just about every other imaginable way. It looks better, is more comfortable, handles better, feels better, is usually faster, etc.
Nothing is 100% sure on reliability, but 3-series BMWs usually do not become money pits. Just be warned that when it comes to fixing them, parts tend to be a bit more expensive then what you might be used to with higher production American and Japanese cars. On the other hand, I have found that BMWs are less likely to need trips to the shop then most other brands.
They are great cars and I hope you enjoy yours.