I fly mostly .40-to-.60-size sport planes, and I find that the 6V NiMH receiver packs work well for me. I've been buying a fair number of the 1600Mah Duratrax packs from Tower to use with my airplane receivers. When I first switched to 2.4Ghz, I used the regular old 4.8V 600Mah NiCd receiver packs I'd been using with 72Mhz radio systems. My Airtronics RDS8000 and my 92824 receivers worked flawlessly at 4.8v.
The price of 6V NiMH packs has gotten so low, however, I've been buying them instead. The Duratrax packs are $14.99 before any discounts, which is cheaper than many of the 4.8v 600Mah NiCD packs that are available from Tower.
These batteries can't reliably provide enough current for high-torque or digital servos in any kind of elaborate setup. I'm happy with them because I'm flying them in planes with 4 or 5 standard sport servos. I can show up at the flying field, top up the receiver pack with my Quick Field Charger II, and I'm ready to fly all day.
I still have to top up the batteries before I fly with them, even if they've sat for just a few days. NiMH packs just self-discharge too rapidly to be counted on unless you've just topped them up. Other than that characteristic, I find them to be cheap and reliable.
That having been said, I regard my receiver and receiver battery as the most important components of my aircraft. If I were working on a project with high torque or digital servos, or an aircraft with 7 or 8 servos in use, I wouldn't hesitate to spend the little bit of extra money required for a top quality battery, heavy duty servo leads, and a heavy duty power switch for my aircraft.
Like most purchases in this hobby, it usually boils down to picking the right part for the project in question. Smaller NiMH receiver packs aren't necessarily a bad choice, but there are plenty of applications where they aren't the right fit.