Like I said, pay your money and take your choice.
If you don't really mind the drops in quality of service or the congestion that you will inevitably get from a service like Be/O2 then by all means save yourself money and use them as an ISP. I would go with Be just on the fact its a shorter contract (3 months, not 12/18).
If you require a constantly stable connection for business use with a decent network and low latency and the ability to cope with demands from its users then you're going to have to either a) pay for a leased line b) pay for a business grade ADSL service where the contention is much lower.
My requirements for when I work from home fall into the latter and as such I have two connections, one provided by Be which is great for high-troughput and just shifting large volumes of data and one from Zen which is what I use when I want something I know is perfectly stable. Entanet were always very good for me too, but their recent issues with the BTWC system meant it was no longer suitable for my needs so I left.