A nice way to use the highline package together with Gnus is to turn on the highline mode in the summary buffer. So you always know where you cursor has been in the summary buffer (even if you switch to the mail).

Simply get the highline (HighlineMode or Groups:highline.el) package and use the following setup:

 (require 'highline)
 (add-hook 'gnus-summary-mode-hook 'highline-local-mode)

Another option is to use hl-line-mode which comes with GNU Emacs 21.

 (add-hook 'gnus-summary-mode-hook 'my-setup-hl-line)
 (add-hook 'gnus-group-mode-hook 'my-setup-hl-line)
 (defun my-setup-hl-line ()
  (hl-line-mode 1)
  (setq cursor-type nil) ; Comment this out, if you want the cursor to
			 ; stay visible.

For aesthetic values, there is also GnusStripes, which puts stripes in the mail summary buffer. That is, odd lines are a bit shaded. There is a similar effect in thunderbird.