I think the designs are ok. Nothing flash and I think elsewhere on this site it has been commented that they are a bit out of date (I remember reading something that Jeff_H once said about the Roberts designs along this line). They are good seaworthy designs, however there are better designs out there (Van de Stradt comes to mind).
The other negative is build quality. A lot of Roberts designs are home built and the quality runs from very good to bloody awful. You really need to spend a lot of time checking out the boat plus a good surveyor is a must. I'm lucky that my boat was professionally built.
Now the positives. The main one (especially in Australia) is value for money. These boats are cheap so a good one is going to be a lot cheaper to purchase than a equivalent name brand. For me that meant being able to get a decent sized boat without having to mortgage everything.
I also recently prepared a summary of all the yachts participating in Sail Indonesia, as I thought this would give me a good idea of what people are sailing offshore around Oz. Turns out the most popular design are Roberts; mainly Roberts 434, Offshore 44 and Mauritius 44. (next most popular were Halberg Rassys and Adams).
On my particular design (Roberts 45 Classic) I am very happy. I wanted a heavy boat that is well behaved in rough conditions, something we get plently of here along the West Australia coast. I also wanted a boat that could comfortably fit five persons (2 adults & 3 kids). Have also been surprised at how well the boat sails, however that is partly due to the sail size (1,011 sq ft). The fact that it is a ketch rig means the large sails are more managable, an issue for me as I do not want to scare the family. In WA I have seen a lot of skippers scare their wives / families in rough weather to the point where the skipper sails solo and / or the boat is sold. My boat is very well behaved in rough weather, in fact the biggest issue last time in rough weather was that everyone in the family was asleep and I was looking after the boat by myself.
The negatives on my boat is going in reverse (she is a full keel with a cutaway forefoot, so I should'nt really complain) and fixing all the leaks!
Still she is 28 years old so I should'nt really complain, plus I'm getting ontop of this.
My boat also has an issue with poor use of space, however this is mainly due to the design of the pilothouse. About 15 years ago a previous owner built a pilothouse over the centre cockpit. I am seriously considering converting the pilothouse into a doghouse and fixing the poor use of space issues. When I firm up my plans more I'll post some details on the Designs thread for people's comments.
Here is a link to page that shows my particular design. My boat has the bow from the schooner, the underbody design of the sloop and the inside layout of the "Roberts 45 - centre cockpit" which is shown on another page.
Roberts Classic 45
Also below is a picture taken last year over at Rotto.