Russ,
For bashing and cruising, I don't think there is any way a Nimh pack can compare to a lipo. especially with the advances in lipo performance in the last couple years.
Lipos have capacity ratings in the same way Nimh and Nicad cells do in mah, but then also rate the available discharcharge rating which is called the "C" rating competitive racing requires a high "C" rating like 40C up to 65C this nuber refers to the available discharge amps i.e. a 65C pack will withstand a 65 amp sustained discharge with burst discharge up to 100 amps often times, numbers never even dreamed of with Nimh packs.
With that said though, for bashing in the yard with 15-27T brushed motors I wouldn't worrry about the C rating, you can get very good lipo packs that have a 25C rating that will outperform your Nihm packs both in speed, run time, and overall pack life. A lipo pack can be cycled as many times a day as you like unlike Nimh packs that need rest to be at top performance. The power delivery is not nearly as digressive as a Nimh pack, meaning 5 minutes into a lipo run you're probably gonig to be still very near peak pack voltage, whereas a Nimh pack will have dropped in the mid 80% range. And for whatever reason, run times with lipos are always longer than with and equal mah Nimh pack, I think this has to do with the way they discharge, and how as Nimh packs get warm the internal resistance goes up and you start losing lots in heat energy. As far as the lifetime of a cared for lipo, it's usually hundreds of charge cycles before any cell degradation starts.
I'm not sure if you know the basics of lipo or not and how the cells work, but all the 1/10 scale lipos are 2 cell, and lipo cells put off 3.7v per cell making a pack that is 7.4 volts. I know lipo tech was confusing for me when I started, as it is for others, all the talk of balancing and fires makes it down right intimidating compared to putting a discharged pack on my peak charger.
Like any pack, the lipo cells need to be equal or close to equal in voltage hence the balancing. Balancing is achieved two different ways usually, a "Balance charger" or an "External Balancer" I use an external balancer by Common Sense RC, it balances the pack by discharging the overcharged cell at .1 amp until the cells are equal and it has an LED read out that tells you the cell voltage and will work with any Lipo compatible charger.
http://cgi.ebay.com/Common-Sense-RC-Balance-Pro-Lipo-Cell-Balancer-Tester-/290481266357?pt=Radio_Control_Parts_Accessories&hash=item43a2085eb5
The other option is to use a "Balance Charger" (I plan on switching to this style charger for next season) and really this is probably the best option. what a balance charger does is charge the cells individually bringing them both to and equal peak voltage. The most popular in the pits is usually the Hyperion Series chargers. It's a little expensive on the front end, but it will be the last charger you ever buy.
http://cgi.ebay.com/HYPERION-EOS0606I-AC-DC-BALANCE-CHARGER-6S-6A-50W-/120634768381?pt=Radio_Control_Parts_Accessories&hash=item1c16647bfd
Running a lipo in your car is pretty much the same as running a conventional Nimh pack with the one difference being that lipo packs don't like to be over discharged, sometimes if you over discharge a pack they can be damaged. A lot of speed controls now have a lipo cut off, or you can use a cut off device like a novak smart stop:
http://cgi.ebay.com/NEW-Novak-Smart-Stop-2-Cell-LiPo-Cut-Off-Module-NIB-/390255062016?pt=Radio_Control_Parts_Accessories&hash=item5add03ac00
Personally, I just keep tabs on the time I've been running and check the pack voltage now and again, the bottom line is the packs don't want to go below 6.0v or 3.0v per cell. Most chargers now will tell you the pack voltage, as well as how many mah youve used, so you can gauge how much time you can put on your packs, Think of it the same way as watching how much fuel you've used in your car buy watching the trip meter and then calculating the mileage buy how much fuel you put back in at the pump.
Lots of people do the "oh fire" dance when talking about lipos, the only lipos I've seen catch fire (one since they've been at the track) or heard about catching fire, have been abused, damaged, mis handled, mis cared for, etc. Granted, like any battery you want to take precautions, don't charge them unattended, store them properly, charge them properly, if one gets damaged be extra careful when you attempt to charge it, etc. Just use common sense.
There's lots of info out there about them if you choose to go that route, like I said, outside of the initial investment expect about $180 for a very good charger like the Hyperion and a high quality 6000mah pack like the SMC MAX (a 6000mah lipo will give you more run time than you ever imagined, when I bash with my son these are the packs we use, and we often get bored before they need to go on the charger):
http://cgi.ebay.com/NEW-SMC-LIPO-BATTERY-6000mAh-2S-7-4V-28C-6028LD-/220705468712?pt=Radio_Control_Parts_Accessories&hash=item3363123128
If you follow the directions it's all very easy and you get the hang of it really quick, the technology is just new so it's a little different, that's all. Hope that helps.