I've got an Oregon rather than the Dakota, I went for that as I've been using my mobile with memory map installed fr a number of years.
I decided to change as the phones weren't waterproof and died fairly regularly, plus the battery life wasn't great with the GPS.
I haven't fitted the unit on to the bars as I crash a bit and worried about breaking it (Although the unit does seem fairly tough).
You can't put memory map maps on to the Garmin device so you'll need maps. I got the Discovered GB Full in a bundle, some will point out that you can put other maps on, which is true. However I ride all over the place so having the UK in full is great.
The good things I have found are; battery life (more than 13 hours while out riding on rechargeables), uses AA batteries, so you can easily pick up some from a local shop, GPS al the time (mobile wouldn't last too long with it on), and its waterproof & tough.
I wouldn't say its a direct replacement for your 205, more of a replacement of the map and compass for the majority of rides.
Oh one other thing, memory map can talk to the unit just as it can with your 205.