Ok...
First off they are fun as a shell ejecting plinker, the action is a good giggle . There are 6mm ...and piss chaep forwhat they are .
Mine Chronoed on a Madbull chrono at around 270 fps which is also what Kermit found . Greasing the seals on the "bullets" increases this a little but at the end of the day you will struggle on such a cheap bit of kit to do any better for an air seal , the air nozzel is not replaceable or upgradable. Neither can you pack the spring or put a larger one in there becasue these is no room for it to compress, and frankly even if you did achieve it the monkey metal would last very long.
On the field ...it's a non starter .... unless you are mad ( as is Porta )... trying to snipe with one up against AEGs is a nonsense ( still didn't stop him trying though 
)
Wanna Kar 98 ...fork out for the gas Tanaka , it's hands down one ofthe most effective bolt actions airsoft weapons of all time ...consistantly chronoes at high amounts and is reliable (ish) .Or get a K98 N/O stock and build a VSR / clone into it.
Dboys shell ejector is great fun , but would recommend it as your first or first choice of weapon .
alles klar ?
funny, this is the same information I gave you on Arnies.
the tanaka has 2 types, the G spec and the export versions. the Gspec is limited to 330 fps, i believe has nossels which regulate the flow of gas and the export is unregulated, which is the one exported outside japan, as the japanese will only allow 280 fps from their weapons. add to this the fact that there are the tanaka KAR98K and the mountain trooper gew 33/40 ( the short carbine version used by jager baallions and also for a small period of time the fallschrimjager)Price as you probably know is £400, more for the Gew 33/40. problem is now its hard to find a stockist which has them in stock. Wolf I believe may have some. They are the porsche of the airsoft world. grant you I am bias, as I love them and have been found to be walking around the house stroking mine. 
On the export version, at Husky it was crono'ed at 470fps on 134a. there is very little to compare to them.
but if you want something which is just for plinking then Old Un is correct and the Dboys is fine, but as anysort of skirmish weapon, forget it,it won't cut it. to be honest buy a luger by WE, cos it will out perform the Dboys.
They are well built. Like every other airsoft gun It depends on what you are going to do with them. It's expensive and needs to maintained.
As for upgrades, some on here have changed the hop up rubber (best gun rubber,I think). In its stock form it's very good.