I've got a Kenwood TS-480 that is currently being used as my base station and was bought with the intention of mobile use when/if I decide to move everything from the shack to one of the vehicles for a trip. I'm kind of with Jon as far as maybe using a 102" whip and a tuner, but I haven't decided exactly how to set it up or if its even worth it right now. Purpose built HF mobile antennas are just too dang expensive for the lack of durability and can be a bear to fool with if you're moving a lot. If I had the extra cash to blow and do some testing, I'd actually like to try either a Little Tarheel or a Diamond SD330. The SD330 is roughly $150.00 cheaper than the Tarheel, but only goes up to 10M, which isn't a deal breaker for me since I don't use 6M much, if at all now. Since I'm using NMO mounts on my Tundra and 4Runner now, I'd likely just get an NMO to 3/8-24 adapter, ground the hell out of the trucks and then see what happens. The 4Runner would be the biggest PITA since its got a Diamond K515 rack mount, but it is still something on the bucket list to experiment with.
@Wdfnds the traveling is doable, but rag chewing like you can on 2M/440, not so much.... Propagation plays the biggest role in HF work and isn't all that great ATM anyway, so adding a moving vehicle into the mix likely won't work very well, if at all. You also have the problem of tuning around. Its not like UHF/VHF where you can program frequencies for repeater and simplex into the memory and then just pop to or scan through them very easily. The bandwidth is just too great on HF and what's open now might not be 30 seconds in the future. You pretty much have to tune around the bands, which can only safely be done while sitting still. ETA: unless you were doing something like tuning into a net on a specific frequency at a specific time and even then the propagation at the time will still play a huge part in what you can do.
@Michael G. , DO IT! IMO, its better to have it and not need it than need it and not have it. The General test isn't hard at all and the upgrade will give you access to much more of the spectrum than Tech. Extra, on the other hand, is a whole other animal. I'll get Extra myself at some point, but other than the prestige of having the top class, I don't feel I'm missing out on too much bandwidth. There's more than enough to keep me happy for now.