I've used OTB beach once. The search engine is very good at seeking out what is available, but I didn't like how it can take them 24 hours to book your flights (what if it sells out or the price increases?).
It also adds a layer of complication which is likely to cause difficulties if the flight is cancelled, I'd rather have a direct relationship with the airline. I think they just book with whatever scheduled/budget airline flies on the route you're asking for.
Friend has used them a lot and she likes how you can pay in instalments over time, but we never book far in advance to benefit from that and I'd rather pay on a credit card for the extra protection, plus I'd just transfer the balance to a 0% offer if I needed to spread the cost.
There was a small discount on the flight + hotel package compared with booking separately, but I don't think I'd bother again.
I suppose it depends what you are looking for and what is available as to whether they beat other operators on price.
I've just entered the dates for the holiday we booked the other day with jet2holidays and OTB aren't offering anywhere near the same price, it's quite a bit more.