There are varying degrees of quality in slimline kits.  The best I have found are the Artisan slimlines from Craft Supply.

The writing ability depends a lot on the ink refill.  If you want an upgraded refill, call Ed at Exotic Blanks dot com and talk to him about what he has available.  I have also purchased genuine Cross refills from Colorado Pen Supply.

For a really nice writing pen, make a pen with a Parker-style refill.  The styles are limitless, in both click types and twist types.  Again, Ed at Exotic blanks sells an upgraded refill, although the refills that come standard in the Berea-sourced kits (bereahardwoods.com) that are resold by many of the suppliers are of a pretty decent quality.