I've used the Lowrance 3500C and MapCreate software for a month. Love the unit - huge, clear display, easy to use interface, options up the wazoo...
The Mapping software itself is easy to use. Waypoint/route creation is fast and easy - and using a 1G SD card is so COOL! I've got NM, AZ, CO, UT, and NV loaded at 1:24000 topo level w/ 120K space left for user files.
Here's the beef:
The cartography is no where as complete as the Garmin. The Lowrance maps seem to be about 5 years out of date on several major highways, and even tho it's supposed to be 1:24000 VS 1:100000 for the Garmin, I've already found it's database is lacking detail. Thats not to say that the Garmin is flawless by any means - it's got some serious errors too - but it seems the Lowrance is worse by a fair margin. One item that I've used for visual reference in the SW are the high tension powerlines. In most cases you can spot'm from a very long distance, yet Lowrance doesn't show them on the maps. That's a bit shocking to me since the unit is touted for use as an avionics aid to nav.
I haven't gotten a reply from my comments to Lowrance on the cartography problems. If I do, I'll let ya know. I mentioned to them that I'd gladly trade off all the Point of Interest database (it's huge and quite complete) for accurate maps.
Mark