I've got an early-build (October 04) WK Limited with the hemi. No reliability or safety problems. There were a couple of minor issues that have been taken care of through TSBs (poor radio reception, foglights staying on when automatic headlights are used are two that come to mind). For a complete list of TSBs/customer satisfaction notices/recalls go to wkjeeps.com and look for the TSB tab.
If you're considering downsizing in the hopes of getting better gas mileage, don't bother...the Jeep may be smaller than the Tahoe, but the gas mileage isn't better. And the Grand Cherokee isn't that great for interior space...the Tahoe has a lot more interior space.
The hemi is a great engine...powerful, smooth, responsive, and actually reasonably economical on the highway...not hard for me to get 18-21 mpg on the highway depending on speed and conditions. 18 mpg highway if I've got it loaded with people and stuff and drive like I've got a purpose, 20-21 if I'm by myself and cruise at the speed limit.
For me the WK was a good choice...nice size for me inside and out, like the power, I like how it drives, and the gas mileage doesn't bother me. I didn't want anything larger than the WK since I drive every day in horrible traffic and need something that makes me feel safe yet is still small & maneuverable enough to let me move easily through traffic, while still having enough space for me, my dog, and friends to take off for a weekend. The WK fit the bill.
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2009 Aprilia Dorsoduro 750
2008 MV Agusta Brutale 910S