I think Tom was perfectly used! He had some good episodes with him at the centre, but also was a flexible character (being the nurse etc.) He didn't always seem to be the focus of the show, but had his moments!
He wasn't annoying, but he had a quirky persona. I loved the way he was into 20th century Earth (especially all the Holodeck stuff and the random quotes). Also, I loved the way he was like a big brother to Harry, and was really good with children
His relationship with B'Elanna was good, and they are my favourite Trek couple. They both had troubled childhoods, so this helped them be able to comfort each other.