We've been using the Fulcrum app to do our data collection for the past two months and we've found it to be versatile, easy-to-use and really flexible in terms of what kind of data we are collecting. We have not had a chance to use it on invasives yet, but that is coming up this up this month.
For instance, we had to collect infrastructure data (signs, culverts, fences, retaining walls) along a 10-mile section of hike / bike trail. We used the web interface to create a form that allowed us to capture specific information about each piece of infrastructure (150 unique features including multiple pictures) and we did it in a day (we were on bikes!) We used a cheap Samsung tablet and a bluetooth GPS unit as the hardware components. At the office we downloaded the data and dropped it in to ArcGIS for use.
Pros: Cheap subscription ($18 / month on a month-to-month basis); really sophisticated and versatile form creation; any type of hardware on either Android or iOS; picture data tagged to locations; you can create attributes using Excel-like functions
Cons: point features only (kinda like MISIN in that you would need to specify areas of invasive populations); subscription auto-renews (so pay attention when you are done for the season!); adding a basemap to an offline connection is convoluted and cumbersome (this is overcome with a cell connection - we didn't have one.)
If anyone is interested in collaborating on a form for invasives please contact me. We can use the RC&D's subscription for the season and grant your group access to use the app as a test case.