Touch screen is a little sluggish

Have you tried recalibrating the touchscreen?

To calibrate the screen of the Zumo 550, press and hold the power button for about a minute until the screen calibration appears.

master reset: with the Zumo off, press andhold the lowest right corner of the touch screen while powering it on, keep the touch touch screen pressed until it says something like are you sure. then press yes and follow the onscreen instruction

I have had Zumo 550 for 2 years now, and it does exactly the same.
I think it is just the way the unit is, and is not a fault.

I am sending it in for warranty repair.

Has the unit ever been really wet or is there a hole in the touch screen foil? If not have you ever done a cold reset?
. First turn the unit OFF, then HOLD your thumb on the furthest bottom right hand corner of the screen, push as far into the corner as you can on the screen itself. While holding your thumb/finger on the screen press and hold the POWER ON button until you see the ZUMO splash screen, once the splash screen is up you can release the power, but continue to hold your thumb in the right corner of the screen for another 5 - 10 seconds, you should see a window that pops up and asks "Do you really want to erase all user data?", make sure to answer YES to this window, if you don't see this window, then turn unit OFF and try the above instructions again, ensuring your thumb is in the furthest portion of the bottom right hand corner of the touch screen.

The problem is with maps overlaying. If you go to the little spanner and then press Map and press Map Info you should only see one Map info. If you have more than one then you need to connect to a computer and go to the Garmin drive. Remove the additional map this will be called suppmap1. Just move it for the time being to you computer just in case you need to put it back.
Another way to check is to press and hold the Volume key on the left whilst powering up. This bring all the info for the unit. Press Next then More on the screen the map name is the 6th one down.
Whatever Garmin tells you to do will not work. I sent mine back and they replaced it within 5 days. Service on that score is excellent.

Do you see a map, with street detail? Any SD card installed?
Tap the wrench symbol, tap mapping, tap about, any maps checked? if not, check the apropriate map and go back to the map screen

I'm not familiar with the Zumo family of GPS from Garmin, but if they are similar enough to the StreetPilot, you should be able to recalibrate the screen. In the StreetPilot series, to recalibrate you turn the unit off. Press and hold the bottom right of the screen and turn the unit on. Keep pressing the screen until the calibration information begins. Try this, see if it works on the Zumos, too.

I have recently purchased a Zumo 550 and successfully copied across selected phone book entries from a Nokia N95 without the need to use any additional apps:-

1) Assuming the N95 and Zumo are paired via BlueTooth
2) In N95 Contacts - use 'Mark' / 'Mark All' as necessary to select the contact entries you wish to transfer to the Zumo 550
3) In N95 Contacts - select 'Options' then 'Send Business Card' then 'Via Bluetooth'
4) Select the Zumo 550 device from the list of paired Bluetooth devices
5) Contact data is transferred from N95 phonebook to Zumo 550 phonebook

Interestingly, Garmin don't seem to list the Nokia N95 as being a supported device on their website, but the above process worked for me!!

There is an ISSUE with the DEAD ZUMO, however some of the are solved, and some need to be shipped back.
There are a couple of options.
1) remove the battery, keep power button pressed for a few minutes. install battery, try.
2) fully recharge the battery with the wall charger, try
3) With a fully charged battery, press and hold the + button and press the power button(hard reset).
4) hold thumb firmly on top left of touch screen and hold power button till it switches on(hard reset).
5) Try a different battery (fully charged)
6) Ship it to Garmin