I bought the biggest Ansmann I could get, the 2850's, and they're great. I tried the 2500's, but they are marginally longer, and they spread the carrier apart too much.
The design of the standard battery carrier is appalling, especially the early carriers using springs on the + terminals. I also found that the carrier tends to want to spit the batteries out a little, so I always use masking tape around them, and this solved 99% of the batteries appearing not to work.
Because you can't charge with the batteries in situ, both the carries, and the battery insulation deteriorate over time, causing more unreliability.
The new rechargeables should solve all these issues, but I'll see how long they last, 2300 milliamp, compared to the 2850's I've been using.