I can't say if the FiOS DVR is better or worse than your current cable company's DVR since I am in Comcast land however according to at least one person in the quote below that's probably the best comparison so far.
said by WaitinginNH:Well coming from Optimum to Fios I think the STB is a huge improvement. Much, much faster than the cable SA boxes. As far as the DVR, it's performance doesn't seem any worse than the SA box with cable, but it doesn't always allow me to rewind or pause a program when I'm watching it, I dont know why that is.
Everyone's experience and comparison will differ from what they are coming from. If you've never used a DVR before, any DVR will appear awesome. My best advice to you is to try the service out because you can always go back to cable. Don't let your decision to try out an entire service be determined by only the DVR. DVRs can be replaced and even 3rd party DVRs like TiVo can be used if you want to go that route.