Still active bug in 5.0.2p1.
Now enters two chars ) and space -- ") " when pressing tab.

I've used this solution for tab navigation (found on forums):

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
using UnityEngine.EventSystems;

public class InputNavigator : MonoBehaviour
{
EventSystem system;

void Start()
{
system = EventSystem.current;// EventSystemManager.currentSystem;

}
// Update is called once per frame
void Update()
{
if (Input.GetKeyDown(KeyCode.Tab))
{
Selectable next = system.currentSelectedGameObject.GetComponent<Selectable>().FindSelectableOnDown();

if (next != null)
{

InputField inputfield = next.GetComponent<InputField>();
if (inputfield != null)
inputfield.OnPointerClick(new PointerEventData(system)); //if it's an input field, also set the text caret

system.SetSelectedGameObject(next.gameObject, new BaseEventData(system));
}
//else Debug.Log("next nagivation element not found");

}
}
}