The $4.99 is for the continued development of software for the DVR as well as the data necessary to operate the DVR, which goes beyond the guide info needed for standard receivers. Stand Alone Tivo's are much more expensive, I think it's about $12 per month and there are options that cost more.
I've had a DVR for about 18 months now and in February replaced my other 3 receivers with DVR's for a total of 4 now. I still only pay the one fee of $4.99 a month. It's WELL worth it. I have no complaint about the fee and after you've used your DVR for a short period of time, you'll wonder how you did without it.
As Sports Illustrated said... "It's the best invention ever, period."