What do 了 and 好 indicate in ...别人用了我们的赠品，觉得好...?
Does this sentence indicate something about completion of an action or something in the past?


'verb + 了' indicate the verb is completed, but it doesn't indicate any tense, It can be in the past or in the future. 

Example:

昨天我(吃了)就睡 - I slept after I ate yesterday

現在我(吃了)又要去睡 - now I've eaten, I have to sleep again

明天我(吃了)還是要去睡 - I will still sleep after I've eaten tomorrow.

In all three sentences, 了 only indicate the action 吃 is completed. 


  别人用了我们的赠品，觉得好


'If' is implied.

如果(if) 别人(people) 用了 (used) 我们的赠品 (our gifts)，觉得好 (consider it is good) 

别人(people) 用了 (used) 我们的赠品 (our gifts)，如果(if they) 觉得好 (consider it is good)