Shelby,
I have three Dolphin cleaners. Two old-as-dirt Diagnostics, that I bought used from Craig's list, and a new Premier that I bought a few weeks ago from Pool Express. The two older units are still working fine in my daughter's pools and Premier is in my pool. So far I'm quite happy with it, but as I said it is only a few weeks old. I've been really happy with the Diagnostics, so I expect the same out of the Premier. Some of the differences I see are:
1. The Premier is about twice as fast.
2. The Diagnostics have one drive motor and although they wander all over the pool and do a good job of cleaning, the Premier either has two drive motors or some other way to control the tracks, because it can make very sharp 90 degree turns that the older units cannot do.
3. The older units come with a very thick and heavy bag (which I like). My Premier came with two sets of interchangeable filters, four fine screen filters and four cartridge filters. It also came with an optional bag filter similar to the older units, but much lighter in texture. The bag filter includes a spare door assembly, so you can simply swap between the bag and cartridges with very little effort.
4. The older units will run for one cycle or about 6 hours and shut off. You have to manually turn them back on when you want to run them again. The Premier has a built in timer system that lets you schedule the unit for a week. You can set it to run once a day, every other day or three times a week. You have to restart it once a week.
5. The Premier has a light that tells you when the filters are dirty. I'm not sure if the light is based on time or if it really can tell if the filter is dirty. My light has come on and the filter did not appear to be all that dirty.
6. Both of these units will climb walls with no issues at all. The Premier has four white soft rubber pads to help it climb, but I can't really see much difference between the older units and the Premier.
7. Both units have bottom access to the filters. I can't see where this makes any difference with the easy of filter maintenance, but top loading units do have the ability to have a third brush in the bottom that mine do not have.
8. The older units do not have an option for a remote. I got the optional remote for the Premier, but have only played with it for a few minutes and believe that unless I'm really bored, it will never be used.
9. The cables for the older units are tired directly to the robots. Over the years they have gotten a lot of kinks and tangle easily, but I have never seen this effect the way they clean. The Premier has a swivel, which appears to work quite well, but time will tell if this really solve the tangle issue or not.
10. The older unit from Craig's list came with no warranties..

The Premier came with a three year, not pro-rated, no hours or cycles limited warranty.
If you have any specific questions, just let me know.
Thanks,
Jim R.