I made this transition about 6 months ago after having been a DISH customer for the previous 10 years. As others have stated in this thread, how happy you will be with a switch will boil down to the programming you are interested in watching.
One word of caution about programming. In the six months that I have had Verizon FIOS TV, they have not added a single HD channel to the lineup. I expected Verizon to be similar to the satellite providers in terms of adding HD channels, but that has not been the case. If there is an HD channel that you currently receive and cannot live without that Verizon is not carrying, do not count on them adding it anytime in the near future.
In my situation costs were about the same with both providers. What appears to be a great deal with Verizon is quickly eaten by the monthly fees they charge for the receivers ($20 for the DVR, $10 for the HD receiver, $5 for the SD receiver). In my case those fees add an extra $45 to my monthly bill.
Picture quality is better on some channels, worse on others.
Things I like about FIOS TV better than Dish
1. Whole house DVR works as advertised. One DVR that can be accessed by every receiver in the house. The limited 20Hr recording size is offset by #2.
2. On Demand library, pretty much everything that you would record to your DVR is available on Demand the following day.
3. Media manager, which allows you to view your video content stored on your computers through the verizon receivers, is nice.
4. The utility for accessing the DVR from the web works flawlessly.
Things I don't like FIOS
1. The FIOS receivers are terrible. The receivers are slow to register the buttons you press on the remote, so slow that you will press them over and over and then watch the screen scroll passed the channel you wanted.
2. The channel layout is cumbersome. The concept of .0 and .1 for SD and HD has not been implemented. The HD channels are always the SD channel +500.
3. The dish concept of pre-programmed guides that show you your subscribed channels and the hd only channels is one that I miss a lot.
4. The FIOS receivers don't allow you to receive over the air channels.
5. While all of the receivers can playback content stored on the DVR receiver, the only receiver that can schedule recordings is the DVR receiver.
6. The SD receivers cannot playback HD content stored on the DVR. I don't know why, but I was expecting it to down-convert the resolution and play it back. This forces me to record some programs both in SD and HD which eats into that 20Hr limit rather quickly. The HD receivers can play back recorded content both in SD and HD.
7. The receivers don't play well with the HDMI connections 100% of the time. Every so often the audio disappears or there is a loud buzzing sound for a few seconds after changing a channel. The Verizon recommended fix to the problem? Don't use HDMI.
8. FIOS doesn't offer any 1080p programming, not even in the PPV section.
9. The PIP functionality that I had on the VIP622 is not available on the FIOS DVR receiver. I also miss the external hard drive storage option.
10. Can you tell the FIOS hardware is not even in the same league as DISH's (with the exception of its networking capabilities)? I understand that there are other hardware platforms that I could use with FIOS, but the trade-off of losing the things that I actually enjoy about my FIOS service for an additional expense is not worth it.
At the end of the day, it is only TV. Dish and FIOS both have things that I like and things that I don't. In my case, it boils down to programming, and that has me seriously considering a switch to DirecTV.