The other two answers are literally correct: なんていうか... is an idiomatic phrase that may be translated as "how should I say..." but may vary depending on context.
That said, I'll offer a dissenting opinion: without more context it's hard to say, but it's reasonable to guess that the speaker actually has a fairly negative opinion of this thing they're using, but doesn't want to complain or be directly negative about it. This is not an uncommon trend: for example, the word "いまいち" is now by convention understood to mean "not passable" even though it's thoroughly indirect.
As a listener, being asked to complete the trailing sentence, I would assume that the speaker is definitely not having a fun time with whatever it is.
This doesn't conflict with the interpretation offered by either of the two answers prior to this one.