I have used quite a few rank trackers like Accuranker, SERPWatcher, SERPRobot which are really good but always seem to miss 1-2 feature which could make real difference. Some didnt some have a good UI or didn't offer much customization. Enough of those guys, why I bought SERPWatch:
1. Really great UI!
2. Option to add Note & see Google Updates (If say you made any content changes or took a link any day, you don't need to remember each date for every keyword)
3. If the keyword is in featured snippet
4. Most important, you can see Screenshot of SERP & actually see what kind of Ads or Google rich snippets are coming in between.
5. A Client Panel where they can check their rankings.
6. Probably the best support for a rank tracking tool.

Improvements required:
1. The UI is great but it doesn't offer much info in a single glance. If we consider SERPRobot, it allows a low of kws in a single view. No need of scrolling much.
2. Option to assign projects to clients. If I have already added the project in my dashboard, I can't directly assign it to a client. I need to re add the project on client's dashboard.
3. On demand rank refresh (Sometimes you just want to refresh your rankings because you read about a Google update just now) :P

All in all, it's a steal deal! I dont think any SEO should miss this out.