i had a 2000 SSEi with all options for that year. got about 30mpg out of it in my cross country trips and around 26mpg when going to an from work (all highway)
few problems to watch for are water leaks, fuel door rot (what killed mine), wheel hubs, rear shocks, and blend door issues, LIM/UIM gaskets. all relatively easy fixes.
the 2004-05 GXP had the best suspension but worst motor the 3800 series 2 V6'* natural aspiration and supercharged are pretty bullet proof. looks are all about personal preference. i prefer the GXP side cladding and rear bumper but like the SLE/SSEi front bumper.
the GXP also had 2 tone interior as well as carbon fiber and satin aluminum accents to the interior as well as a much nice spoiler. the tails on either are hit or mis and again personal preference.
basically all trims and years had their issues the 4T65-E trans coupled to the V6 models also needs upgrades to include the 4th gear clutch hub, updated valve body and solenoids. none of which to worry about til needed
2005 Pontiac Bonneville GXP
2008 Chevy Impala 3LT 3.9L (9C3)
2000 SSXP (retired) all over the country in a million pieces