Instruction: Generate an answer to the question by utilizing the information contained in the document.

Input:
Question: Does this vacuum do well with carpet?  If you prefer brissells did you purchase the pet/allergy upgrade brushes?
Document: It's unfortunate that I paid $50 for the upgraded pet/allergy brush but I guess that's the price I pay for not believing the other reviews advising how well these Neato's work within the first few weeks/months... And then sharply drop off in reliability. It's sad. This product is incredible, if only it reliably worked for longer than a week.
Understand that I am a Neato fan - and will remain one. This is my third Neato, but this model is very quirky compared to others and the suction not as good. Will try the newest version or revert to my older Pet and Allergy model.
The brush too is not properly designed for my all tile floors, and unless I spend $50 on the pet-upgrade kit, will continue to miss things.Unlike other robot vacuums that tend to work in a linear back and forth type pathway, the XV-14 wonders around in a sort of drunken-sailor gait. It tends to drive in a straight line for a bit, then in the middle of a room, turn randomly at an angle and drive off somewhere else.
However the new firmware Neato bumped into the chair once, then rotated around making a new map and carried on its way successfully (including docking later). It also apparently deals better with rug fringes, but we don't have those to worry about.We also purchased the pet/allergy brush and filter. The pet brush picks up more cat hair on rugs, and it is also much quieter on hard floors - the original brush sounds like a motorboat on hard floors.
So the dock failures don't upset me. Battery life is ok. I have half-shag carpet throught my main rooms and tile in the kitchen/dining room. It will run for 30-35 minutes before returning to the dock. 2.5 cleaning cycles does my whole house. Someone with smoother carpet or hard surfaces would certainly get more runtime. If Neato ever put out a Li-ION battery/charger upgrade kit, I would buy it right away.
The Roomba's strength here is that it runs for up to 3 hours vs. Neato's 30-45 minutes, but their seemingly random movement means it takes about that long to cover an entire space. The Neato did a great job picking up pet hair of my hardwood floors. I also purchased the "pet upgrade" that effectively turned one of my Neato's into a XV-21 series. I have to say that I saw no appreciable difference in it's ability to pick up pet hair.
It worked well in our open floor plan condo. However the Roomba itself needed a lot of TLC. For example the brushes and ends needed daily cleaning to remove fluff and hair that would wrap around, and if not removed the Roomba would stop and bleep error messages that it's brush was stuck. After a few months, the plastic gears would strip and the brush head module would need to be replaced.
Update after 3 months of use:Bertha really keeps the house clean. In the basement you can see the difference very well because there's a light colored carpet, and as stated she can't clean closer than about an inch to the wall, so now I have a beautiful white carpet with a 1-inch black border around it...The magnetic strips and tapes I bought off ebay (intended for refrigerators, crafts, whiteboards etc.)
I purchased one of these for my 700 sqr ft apt, have a dog and a cat, and I can proudly say the Neato does its job well. I really run this thing hard, too. 7 days a week, 2 times a day. It picks up a full bin of pet hair and dust every day.
I purchased the Neato XV-14 as well the upgraded pet brush/filter. I received the product and set it up (and charged) as per Manufacture's instructions. The product worked great for 1 sweep of the house, and then it went back to it's charging station. Ever since, I've had sporadic results.The robot went back to it's charging station, and then never continued on with the cleaning of the house.
Answer: A1: in my experience (almost 2 years now) yes, this works GREAT with carpet.A2: I have used both the pet upgrade and the original brushes for these past 2 years.  I can't say there is a noticeable difference in performance.  The biggest difference is that the pet brush needs to be cleaned semi-regularly because human hairs get wrapped around it, whereas the original brush requires virtually no maintenance.  Perhaps the pet brush is better on carpet- I'm not sure.  On our hardwood floors it makes no difference, and I have 2 cats and a dog.