I'll add a couple of cents onto Platinum's post to make an even dime

My girlfriend is Bulgarian, and like Platinum, I have elected to stick with Russian (I was learning Russian before I met her) because there just aren't enough resources here to learn Bulgarian.

For the most part, she can understand about 90% of spoken Russian, but she can't (won't) speak it because she isn't sure of the word differences between the two languages. If I ask her what a Russian word means, she will tell me without thinking, but she can't do it in the oposite direction (usually)

Also, like someone else said, the verbs are relatively similar, but with different endings. For instance,

Ya znayu = Az znam
Ya govoryu = Az govorya

Overall, I think the best comparison would be a person that speaks Portuguese listening to someone speaking Spanish. They can understand, but wouldn't be able to speak it very well.

-Fantom