The funline series is simply a slightly lower quality slimline kit. The plating is a bit thinner, and the parts are from a different manufacturer.
These are available at as low as $1.25 per pen for 100+ from PSI, I think, and they are very popular for "Pens For The Troops" and "Freedom Pens" projects. Personally, I think the gold plated funline is better looking than the slimline version ... doesn't have that black enamel rectangle in the middle of the clip.
As for reliability .... only ever had a single one fail me, and that was because it was dropped on the writing tip while the refill was extended. Pretty sure no pen would take that kind of abuse kindly ...
I've only had my lathe for about a year and a half, haven't really had enough time with these to see if their plating will stand up to constant use and abuse over time, but I hear decent things about them.
I'ld say 20 dollars for a funline pen isn't a bad price ... quick turn with a friction or wax polish, I could make 2 per hour. The are excellent, pricewise, for getting kids and students into turning projects ... mass gifts for friends and party attendees or advertisement sources... For additional extras, like a CA finish, stone or metal inlay, or a specialty blank, I'ld definitely charge extra.
The refill is the EXACT SAME as the ones used in the slimline style pens, by the way .... cross style refill